Spyware is a term applied to a variety of programs. The common thread is that they all keep tabs on how you use your PC (in most cases, they track the Web sites you visit, and some collect the information you enter on those Web sites). Many of them use this information to build a profile on you, and then use that profile to present you with pop-up ads.
When spyware first started showing up, it wasn't usually all that harmful. It would annoy you with ads, but it usually didn't cause your PC to crash, and it wasn't too difficult to get rid of it.
These days, spyware is becoming much more aggressive. Many spyware programs, once they're installed on your computer, will download and install more spyware without telling you. Some of the spyware is designed to be extremely difficult to remove; in the worst cases, the only effective way to get rid of it is to reformat the hard drive and reinstall Windows. This is obviously highly disruptive.
Also, remember that spyware programs track what you do on the Web. This could include private information you enter on Web sites. User names, passwords, bank card information: all items which are commonly entered on Web sites and which could be picked up by spyware.
In the old days, spyware was downloaded and installed by unsuspecting users. Typically, you'd download a program you thought would do something useful for you. Hidden in its license agreement was a statement that you agree to the program monitoring your Internet usage, popping up ads from time to time, and possibly installing additional software on your machine. The spyware authors know that basically nobody ever reads the entire agreement; they simply click the "I Agree" button as soon as it appears.
There are now some spyware programs which can install themselves without your knowledge, taking advantage of bugs in Internet Explorer. So it's now possible to get spyware on your machine without ever having intentionally downloaded a new program off the Web.
If you're getting more pop-up ads than you used to, if your machine is suddenly a lot slower than it used to be (particularly when you first start it, or when you're surfing the Web), if your machine is less stable than it used to be, if you have a new toolbar in your Web browser which you don't recall having installed, if the home page your Web browser shows you when you first start it has changed (usually to a site offering to help you search the Web), there's a good chance you have spyware.
Have a look through the Add/Remove Programs applet in Control Panel. Here are some examples of spyware (note that this is not a complete list):
Go into the Start menu, click on Programs, and look for anything mentioning Gator or GAIN. If you're running a recent version of Windows which only shows you recently-accessed programs, you'll need to expand the menu to show you all programs. If you see either of these, you have spyware.
Just as there are anti-virus programs which aim to detect and remove viruses, there are anti-spyware programs which scan for and try to remove spyware.
Be very careful, however; many of the anti-spyware programs you will see advertised on the Web are, in fact, spyware. If you see a flashing banner ad promoting an anti-spyware program, or if you get a pop-up ad for an anti-spyware program, you can be assured that the program is actually spyware; reputable anti-spyware programs do not promote themselves in this way. Even more troubling is that if you search the Web for anti-spyware programs, or go to some major software download sites looking for anti-spyware programs, many of the results you will see are spyware. And some of them have names which are intentionally made very similar to the names of some reputable anti-spyware programs, so if you know the name of the program you want, read very carefully to make sure you're actually downloading the right program.
If you have an anti-virus or Internet security program from a reputable vendor like Norton/Symantec or McAfee/Computer Associates, and it's a very recent version of the anti-virus program, it may have at least some level of spyware protection. These companies also generally have separate anti-spyware programs you can purchase.
There are also free anti-spyware programs available from a number of sources. The following are all reputable:
Some spyware programs show up in Add/Remove Programs. Most spyware which shows up there can be removed from there, though some such programs will try to convince you that they're not spyware or that you really do want them. Ignore their pleas and remove them.
If you found Gator or GAIN in your Programs menu, there's an icon there with information about Gator/GAIN. Run that program. One of its options will tell you which Gator/GAIN programs are installed on your PC. Remove those programs. Gator/GAIN will automatically disappear a few minutes after the last of those programs are gone.
Use one or more reputable anti-spyware programs to scan your machine. Just as no anti-virus program is 100% effective, no anti-spyware program is 100% effective, either, which is why it's a good idea to run more than one of them.
As mentioned above, some spyware is very hard to remove. You may need professional assistance. If your machine is particularly badly infested, it may not be possible to clean it up; you may have to save your data (burn it to CD) and wipe out and reinstall the computer.
There are a number of things you can do, none of which will guarantee that spyware can't get onto your PC, but each of which will reduce the likelihood. In no particular order:
Some of them only scan the PC when you manually tell them to; others include components which monitor your PC all the time and alert you as soon as spyware tries to install itself. Some anti-virus and Internet security programs include anti-spyware programs, and some do not.
Just as anti-virus programs need new definitions in order to catch new viruses, so do anti-spyware programs.
Check out where the download is coming from; is it reputable? Don't download a program unless you need it. When the program pops up its license agreement, don't just click "I Agree" - read the agreement carefully.
Some Web sites have pop-up ads on them. Some of these ads will install spyware if you click on them. Don't click on them. If a pop-up warns you that you may have spyware on your machine and offers to check, don't do it.
Virtually every month, several new security vulnerabilities are found in various versions of Windows. Microsoft releases patches for them. If you don't install these patches, your machine is still vulnerable. Ideally, you should turn on Automatic Updates (in Windows 2000 or XP), or install Windows Critical Update Notification (other versions), so that you computer will automatically notify you when new updates are available. When new updates are available, install them at your earliest convenience. If you want to check for updates manually, click the "Windows Updates" icon in your Start menu, or go to Tools -> Windows Update in Internet Explorer.
Because IE has such a large share of the browser market, and because it has so many known security holes, the spyware writers specifically target it. If you're using another browser, you're less likely to have problems. Here are some other options: