On-line anti-virus scanning:

Some viruses or worms such as blaster, klez and klez variants can disable the Windows installer 
and/or the ability to install or re-install anti-virus programs.  On-line virus scanning programs
may allow you to clean or delete the virus from your system.  If successful, you may then be
able to re-install your anti-virus program and run the update for it.

You can use the free on-line anti-virus scan program by going to this site:

http://housecall.trendmicro.com/