I don't know if this is the same one but it sounds like one I just disposed of .
The pop up box came with the message something like 'You have been infected with winsterHJKv2011'. Press OK to download the fix- yeah right.
I serched google for winsterHJKv2011 and the solution was to delet file c:windows\system32\systr.dll. you need to download killbox to delete it.
Hope this helps