Since early January 2009, I have been fighting against a tide of malware, viruses, trojans, rogue agents, and shyster software unlike anything I've dealt with in the last 2 years. It's been great for business, but it's no fun for anyone to have to deal with. So here's some preventative measures you can take - even if your system is clean at present, having the tools in place will be handy if you do get infected)-. It will take probably an hour or two, but might take longer if it's really deeply rooted.
1) If you are needing an antivirus software (if you don't already have one, or a Norton/McAfee subscription is running out) then AVG Free is the best solution - it's free, and works better than either of those 2 major softwares! You can easily get it at download.com.
2) The quickest way to run a scan and clean up your system is to download and install Malwarebytes' Anti-Malware, also available at download.com. It's the simplest, fastest and most thorough software I have found for this purpose, and I have used it constantly since "the Plague" descended in January.
3) Also consider getting the latest version of Ad-Aware from download.com. It takes a while to download, install, and update, but it can sometimes find problems that MAM and AVG miss.
Between these three tools (all of them are free) you can be 95% certain you've cleaned up any nasties on your computer. MAM and AA can both be installed safely, since they are anti-Malware, and don't have any conflicts with each other, but make sure you have removed any pre-existing antivirus software before installing AVG (you can only have one AV software at a time!)
Bear in mind that if you have some of the more virulent and malicious softwares on your computer, it will probably fight letting you download, install and scan with these tools. You may need to download the software to a flash drive on another computer, then install it on the infected computer. Sometimes you can't even do this, and the only solution is to take out the hard drive and hook it up to another computer and run the scan from there - that way it's essentially powerless to resist. I can do this if necessary.
That's a lot to throw at you, but it's the approach I've been taking for some years now, and it generally works. You can contact me if you get stuck, or if you don't want to hassle with it! :)