If it won't let you use any Antivirus Software, you need to reformat.
Download all the drivers for your computer's audio, video (graphics), and wireless or Ethernet Internet connection. Put the download files (.exe) onto a flash drive or CD as these will also be lost in addition to all data.
1. Back up ALL data! Everything will be lost!
2. Insert your Windows CD, then turn the computer off.
3. Turn the computer back on, and when the brand name for your PC manufacturer comes up, immediately hit the Boot Menu key (should be F12).
4. Choose to boot from the CD.