Once booting start, select "Safe Mode" from the boot menu.
Scan your hard drive by running your antivirus software. If the program locates an infection, move it to your vault or follow the on-screen instructions. After program completion, if a virus was removed, attempt to reboot your computer and see if this clears up the problem. If not, restart your computer in safe mode.
Restore your computer to a previous point. Click on the "Start" button. Select "All Programs", "Accessories", "System Tools" and finally "System Restore". Choose a restore point on or before the last day your computer worked without error. Finally Reboot your computer to see if it loads correctly.