HiJack This (http://www.majorgeeks.com/download3155.html)
Run AdAware and Spybot and update them at least once a month to get rid of "noxious" things. They're very helpful if you run into a real problem. Having them available on your machine before something happens is a "VGT" (very good thing) but they have to be kept updated, like AntiVirus, so you need to unpdate and run regularly for them to be anygood.
Hijack This is a MAJOR TOOL when things get really bad and the idea here is to have it if and when you need it. You can run it and make a log file of your current setup if you like - recommended if you understand what it's for - but you should NOT ATTEMPT to use it to make changes to your system without "pro" assistance, unless you're pretty knowledgeable. Just get it.
If you want to understand what HiJack This is good for, look at the "procedure" at That Comupter Guy. The critical thing you need to know is that Hijack This makes a log of what's on your machine, and that log can be shown to the experts who can tell you what to do about it. ALL such help resources will expect you to have run AdAware AND Spybot before making a Hijack This log for submittal, and links to get them are also at this webpage.
After you've read the instructions, which are pretty much common for several sites that can give assistance, you might want to take a look at the Tom Coyote Forum to see how it works.
Expect to spend a couple of hours doing research, if you have normal curiosity and if you pursue getting a good notion of where the help will be WHEN DISASTER COMES.
All above programs are free, although there are more sophisticated paid versions if you find one helpful. Donations accepted for the free versions, if you're inclined.
OPTIONAL: If you have *WinXP, you can get the free "Microsoft Malware Remover." It's still being called a "beta" but it's been around for at least a year now. It applies fairly sophisticated methods to look for "bad programs" that might have crept in, including spyware, adware, and some other stuff. It was one of the first to officially declare the **Sony rootkit DRM "malware" and to announce that the "Malware Remover" would take it out. It can be "intrusive," since the default is to run daily, and on my machine it takes close to an hour to scan everything. Most people wouldn't have a problem with letting it run while they sleep, but I'm not consistently off for the same hour every day.
*Mainly for WinXP Professional, although I think it's available for WinXP Home. It may be available for Win2K(?).
** Put "Sony" in the filter box and refresh back 60 days if you haven't heard of this one.