Everything you tell me makes me believe that it is a virus. The "nuts" out there writing these virus, are very shrewd. Somtimes they prevent scans, stop you from installing anti virus, even prevent you from going to sites that can help resolve the problem.
Can you get into safe mode and go to last know good configuration? If you can get that done, you will be able to install an antivirus.
Are you saying that you have to install the A/V, now? You did not have an A/V? Speed Up the Performance of Vista - Vista Forums