There are a lot of viruses out there on Windows. If you are not using an anti virus program, you are probably harboring them as well as a trojan or two.
This isn't a primer on how to get rid of viruses. I have already gone down that road. There's a lot of different ways to clean a computer. Most folks get frustrated that their machines have slowed to a crawl and go out and buy a new one. If that's you and you have a laptop that us under a year old in that state, feel free to send it to me... oh and get yourself a Mac, they don't have the same numbers of viruses. You won't be quite as compatible, and the software is more expensive, but as long as you stay in the Mac Environment you will be much better off. If you're a casual user it frees you up from having to deal with all that virus crap.
On the other hand, if you're a little bit technical, you should look into getting a virtual environment and running a computer in a sandbox. That is how I am writing now. I'm looking at Firefox running on a virtual Windows XP program that is running on Virtual PC 2007 that is running on Windows 7.
Got all that?
I have done all this so that I can stay away from Viruses and Google's snoopy tendencies (They already know WAY too much about everyone and I'm moving away from Google's software) and do everything I need to. When I am done, I click the X box to close the entire environment, tell it no to "Save State" and its done. All the snoopy things are gone. There's no connection to the main machine, there are no worries about viruses and illicit cookies, and I'm completely safe.
If you are really curious, and have an older Windows Machine that you want to save, you can create an image legally from your existing older machine (It is legal as long as you have retired the computer you image - like this one was), install free software like Virtual PC 2007 or VMWare Player, and you are good to go. You just need to have an extra amount of memory over what you normally run on the computer.
If I've lost you, find your favorite 16 year old geek and turn them loose on this and tell you what you want to get done. Get the right software on the machine and you're safe.
I've obviously glossed over everything here, this is basically a very high level executive summary. But that is how to do it. It works well.