There are a couple things you can try. If you are running Windows XP you can try rolling back to the last known good system restore point.
Start->Accessories->System Tools->System Restore
You may be able to rollback your system to a point before it was infected. You will probably lose some data that was saved between your last system restore point and today.
If that is not an option, boot into "Safe mode with networking" and download AntiVir. I have had pretty good luck with that. You might also try "Spybot: Search and Destroy" spyware remover, which is free.
Several of the for-fee vendors offer free online scanning tools. Just search for "online virus scan" or "online spyware scan" in Google. Use these tools to get the name of the malware infecting your system and then search for a free removal tool for that pest. Many of the for-fee vendors offer free removal tools for the nastier pests.
Do you have any idea how you caught the pest? If you use IE for browsing, you might consider using Firefox or Opera.
I have written an article about spyware that you might find useful.
http://www.newbiesecurity.com/spyware.html
