Uninstall AVG, Windows Defender and Zone Alarm and install this instead:
http://www.microsoft.com/Security_Essentials/
Update and scan it full. Remove any virus it can find.
Now, click start>run> type 'msconfig'
Unselected all in the startup tab but leave only anti-virus checked from the one above.
Clean the registry with CCleaner. Keep cleaning and remove the files until it's gone.
Disable System Restore then enable them back which will delete all the backup it saved which slow your computer down.
Reboot your computer, it should be faster.
And not last, defrag it and sit back. This will take long.
You are done.
