It’s not the viruses you should be worried about
It’s bad enough when viruses and spyware mess with your PC but when virus scanner providers start screwing with you it’s a whole new ball game. On April 21st McAfee released a definition update (5958) which incorrectly identifies svchost.exe, a critical Windows executable, as a virus and tries to remove it, causing endless reboot loops.
Luckily there is a fix tool available which can be downloaded here. Further details on fixing the problem can be viewed here. Failing that you should pop the affected unit into your local IT Pro.