sounds like windows is corrupted from the virus. Many viruses put registry entries in to prevent anything with .com or .exe being run.
If you know the virus name, you may be able to download a removal tool to clear it up and reverse the damage. Simply scanning viruses doesnt repair the registry, whereas the removal tools do.
Go to the symantec.com web site, click on downloads and then click virus removal tools.
Look in the list (quite large) for the name of the virus you had and download it. You may have to use another pc to do this, but they all fit onto a floppy disk.