There is a current rootkit (a virus of a sort) that is basically undetectable at this time creating this exact problem. It started sometime after opening an attachment from email or AIM and it hides itself very effectively from applications looking for it!

Rustock rootkit v 1.2
Z:\NewProjects\spambot\new\driver\objfre\i386\driv er.pdb

Look for this file in windows mini dump logs.

To configure startup and recovery options to use the small memory dump file, follow these steps.

1. Click Start, point to Settings, and then click Control Panel.
2. Double-click System.
3. Click the Advanced tab, and then click Settings under Startup and Recovery.
4. In the Write debugging information list, click Small memory dump (64k).
The file created should appear in the root of the drive where Windows is installed.

Symantic has a way to maunually remove it follow the directions :

Backdoor.Rustock.B Removal - Removing Help | Symantec

Good Luck!