Help - Spyware detected 37 Trojans, 26 Backdoor agent

[Agent367]

Is there a way I can discover who has been infecting my PC?
Could my PC still be infected????
From 8/18/08 to 10/16/08 my computer was regularly infected and became virtually unusable until 10/17 when all invasions stopped.
Started with Trojans.
I have a briefcase on desktop containing personal and work documents -
9/28/08 I tried to modify document in briefcase, was told unable to - pop up said The briefcase is in use on another computer. Close the briefcase on that computer and try again. I was at home at the time???
10/1/08 Another Pop-up said the briefcase cannot be opened because the disk is inaccessible but I never tried to open the briefcase
10/15/08 detected 14 Backdoor.agent.lel
We have reason to believe someone broke in my work between 11:30 pm and 6:00 am on the 16th as one of my important documents was the only one on the network modified at 1:51 am. Reported to police, cyber detective division for state unable to allocate resources to assist.
10/16/08 am did virus scan and detected 12 more backdoor.agent.lel
10/17 on - all virus scans clean except adware
Any information provided to assist with discovering who did this will be greatly appreciated!!Side note:
Spyware Dr. detected the 37 Trojans and 26 backdoor
Norton detected 1 Trojan on 9/22, successfully quarantined
3 on 8/29, Trojan infected c:\system volume information_\restore_(series of #s), action taken: leave alone, unsuccessful - clean security risk failed. Quarantine failed.

[GroundZero3]

I would make sure you have the system restore disabled. Run adware and spyboy to clean up any files. After you have run both of those pograms run hijackthis

HijackThis Analyzer & Tutorial

[Telos]

turn off automatic updates on your computer, that is a guaranteed back door into any computer with out a ridiculously good firewall or has multiple firewalls that cover that area.