Hello everyone - my first post !
I had this same problem today on my desktop. Here's what I did to address this problem. and I still have more to do.
I started the computer in SAFE MODE - hitting F8 while PC is booting up (windows XP).
I have a laptop as well so I was able to surf the net and download AUTORUNS from http://technet.microsoft.com/en-us/sysinte...s/bb963902.aspx
I transferred the files over to the infected desktop - using DOS commands and my USB key.
I ran AUTORUNS.exe and saw that there was this PC ANTIVIRUS program that was in the startup directory.
In the AUTORUNS program, it showed the image path - to a specific .exe file in the C:\windows\system32 directory. It was something like "aa....9o5.exe" file. Sorry - deleted it already ... and didn't write it down.
I then restarted my PC and was able to get past this program - and windows loaded. I did however get an Windows load warning.
My next steps are to run a full virus scan. Plan to download malwarebytes and run it like what boopme posted earlier.
I hope this post helps. I'm not advocating what I did was correct. I am just posting how I got past this program.
Good luck !