Oh, it really does sound like you have some malware that wasn't detected by your scan. I suggest you try them again, this time in Safe Mode With Networking. That sometimes keeps malware from protecting itself.
You might want to also download, update and run some other anti-malware scanners, Ad-Aware and Spybot S&D in particular. Sometimes one scanner will catch things another won't.
To get into Safe Mode with Networking:
1. Log out and reboot your machine.
2. When the machine starts the reboot sequence, press the F8 key repeatedly.
3. Select Safe Mode with Networking from the resulting menu.
4. Login. If the malware has changed your password, try logging in as Administrator. By default, Administrator has no password.
5. The machine will continue booting, but the Windows desktop will look different.
6. When you're finished doing what you need to do, log out and reboot back into normal mode.
Good luck.
Note: There ARE free versions of these programs on the websites listed. They just may not be obvious.
Ad-Aware Free (free): http://www.lavasoftusa.com/
Spybot S & D (free): http://www.spybot.com/
MalwareBytes (free) http://www.malwarebytes.org/ (If the program doesn’t run, changing its filename from mbam.exe to something else ending in .exe has sometimes proven effective.)
AVG anti-virus (free): http://free.grisoft.com/doc/2/lng/us/tpl/v5/
Avast! (free): http://www.avast.com/
Kaspersky (free trial) (Seems to dislike installing on any machine with just about any other decent anti-malware, including some firewalls.): http://usa.kaspersky.com/downloads/