Virus (backdoor trojan?)?

My computer was infected with SOMETHING. Random websites and fake alerts popped up, and so I downloaded a trial of an Anti-Spyware that someone recommended in another question. Scanned the computer, it found a bunch of infected files, those were all deleted. After this the computer is fine, until I plug it back into the internet and click on internet explorer, at which point everything becomes messed up again. There are a bunch of fake warnings about viruses that tell me to download software that actually turns out to be a virus.

I have no idea what to do. I think I've actually found the problem files, but when I try to delete them, it says they're in use by another program. (Ironic, eh?)

At this point in my life, this is the very last thing I need. Does anyone know what to do?

Virus (backdoor trojan?)?computer

Try this solution, because the trojan is obviously reintroducing itself to your pc. First don't do trials, you want good programs that you can keep. Download Spyware Doctor Starter Edition and SuperAntispyware which are both completely free (no trials). Install then update both programs. Then disable system restore, reboot, then reenable it. This will remove any and all malware in your system restore points. Now take your computer off the internet and do full scans with each program. After the scans have completed, do a complete online scan of your pc from the Trend Micro Housecall website. It will remove any additional malware from your pc. Depending on the exact malware you are infected with you might have to scan your pc with the spyware doctor and superantispyware in safe mode (after reboot hit f8, repeatedly). Also if you don't have a good antivirus software installed I would highly recommednd the use of Antivir which is free and an excellent antivirus program.