AV and Firewalls are not guarenteed to keep you problem free. They are a good start, but can not completely protect the user from new problems.
You may not need to reformat, fully compromised system refers to a root kit or other severe problem (which you may not have)
Do this and see if it helps.
Turn off computer and boot into safe mode. To do this turn computer completely off, and then turn it back on and immediatly hit F8, once the boot menu appears select Safe mode with networking.
Delete Temporary Internet Files, and Temp Folders (Not the end of the world if you do not do this)
Install and run Ad-Aware and Spybot Search and Destroy. Many times one will install and the other will not (says admin privs needed). Anyway run those and remove everything they find.
Turn Off or disable your AV. Different AV apps catch different problems. Install a 30 day trial of Kaspersky or NOD32. Do a full system Scan.
NOTE You may have to do 5 before step 4, if neither spybot or Adaware will install.
Uninstall the trial AV app you installed. Update and run your AV app.
If they (the AV apps) could not remove a problem, but reported the full name, go to Google and type <Insert Virus Name> removal tool.
Use this app, or outline of steps to manually remove the problem.
This will fix 99.9% of all problem virus related. Your problem (task manager missing) may not be virus related though.
So to fix any system files removed by the AV, or otherwise corrupted/Missing run Windows File Protection (Repair install not needed, unless you can not boot in to the OS)
- Go to start and in the RUN box type “sfc /scannow” without the quotes. This will replace all system files with the correct version if missing/corrupted.
NOTE You will need an OS installation disc for whatever OS you are running for SFC to fix problems it finds.