recurring virus

One month ago a virus took over my unprotected computer.  I purchased Norton Antivirus 2010 and that took care of the problem.  However I noticed my computer started to run very very slowly.  So just a few days ago I uninstalled Norton using their Uninstall Tool.  Now today, I have a bad virus back, I can't even use the System Restore, it doesn't allow me to.

 

So I booted from the Norton Disc, entered the product key, and everything looked just fine.  It appeared to download an update and I ran the complete scan, but it did not detect any virus.  Then I rebooted, and the virus is still there, won't allow me to open any programs and also I see that Norton is not installed.  Now What? - Glenn