I would go to another PC, download the "Windows Defender Offline" tool, and make a bootable CD or USB stick, and run the tool on the target PC.
What is Windows Defender Offline? - Windows Help
You will need to know whether the target is 32 or 64-bit.
After you run the tool, then try booting your PC.
You might also try powering on your PC, and immediately pressing F8 multiple times until you see a screen that allows you to boot to Safe Mode, or the "Last known good configuration".
If you do have a virus, you might also want to use Hitman Pro (Home - SurfRight), even after you are successful at booting your PC. After you run Hitman Pro, reboot your PC.
As always, all standard disclaimers apply.
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