Could be simple

That's a generic Fake Anti-virus System variant. These are often as easy to remove as:- Log in to PC as another user- Run Autoruns and use the option to scan as another user to find the infection (it's usually in the particular users appdata folder. That is C:\Users\username\AppData, or C:\Documents and Settings\Username\Local Settings\Application Data and C:\Documents and Settings\username\Application Data. Note these folders are hidden).- Delete it and rebootIf that doesn't fix it (or you don't have another user account you can log in as) then I'd try either removing the hard drive and plugging it into another PC as a slave, or running a boot CD like MS DaRT or Bart's PE and deleting the files manually.