Sorry to hear this. Take the following steps:
1. Find all your data files (Word documents, excel spreadsheets, TaxCut returns, etc), and back them to CD.
2. Do it again.
3. Find and back up all the keys you have for any programs you've purchased and downloaded. Hopefully, if you're like me, you keep all details written in a little black book next to the computer.
4. From an uninfected computer, download Eraser from here:
http://www.heidi.ie/eraser/
5. Install it on that uninfected computer.
6. Use it to create a NukeBootDisk.
7. Use it as per the instructions on your current computer. Scary, but it WILL totally erase your hard drive, including any "hidden" progams which routinely escape Windows reinstalls.
8. If you haven't upgraded to the latest version of Norton Antivirus, yet, go to the store and purchase a copy (2006).
9. Once the NukeBootDisk finishes, your computer is, as the short lady in Poltergeist said, "is clean!"
10. Reinstall Windows without reconnecting to the network, choosing to reformat the hard drive (doing so locates and blocks off bad sectors). Don't choose multiple partitions. One big one is fine.
11. Install your fresh copy of Norton AntiVirus.
12. Connect to the Internet and ONLY go to Symantec and update your virus definitions FIRST. Not even to Microsoft.
13. Update Windows.
14. Download and run Spybot and Adaware. If they ask you to change anything, delete anything, etc., say NO! It'll educate them as to what's supposed to be there. But do set Spybot to run in the background to prevent Spyware infection.
15. Download and install the latest versions of all your drivers (video, network, sound, printer, scanner, etc.).
16. Finally, create a folder on your desktop called "Old Stuff" and copy everything from your backup CDs to there.
17. When you need something you know is in Old Stuff, create a new folder for the subject on your desktop and drag only the pertinant files into that folder.
After a few years, archive your Old Stuff to CD/DVD using a program that spans CDs and DVDs like PKZIP.
Yeah - it's a royal pain. But I did it in 1999 before I got smart, purchased a NAT-based router/firewall from Linksys, properly configured it to keep everyone out, and haven't had a SINGLE problem since!
Reason - even running Norton Antivirus loaded for bear won't protect your computer from being hijaacked. We tested unprotected computers in Korea, and it took about 15 minutes, on average, before hijaacker's scanning programs detected them and began using them as piecemeal servers for file-sharing services.
It's that prolific, and about 30% of all home computers are infected in this manner. It's not a "virus," however - it's not malicious, or designed to cripple your computer, so it's often overlooked by antivirus programs.
Go here for some older details on how to properly configure your Linksys router. It's about 2 yrs old, but the same principles apply:
http://www.seoulcc.org/meeting_archive_2004.html
Check out the June 04 meeting entitled Linksys Router Configuration.