I empathize, as I have a teenage son who is always infecting our PCs with some type of scumware or other. Now, you've probably already checked this stuff, so forgive me if these suggestions sound condescending.
Did you already check HKLM\Software\Microsoft\Windows\CurrentVersion\Run in the registry for any suspiscious entries? If so, delete the entry and associated executable.
Did you check both your Startup and the All User Startup groups for the same? If so, do the same.
At system idle are there any suspcious processes in say the top 5 or 10 on CPU Utilization? You can check this by pulling up the Task Manager, clicking on the Processes tab, and sorting by the CPU Utilization column (click on the CPU heading). If you do find a suspicious process, note it's name and check Google for a cleaning procedures.
What was the site? Again, you might find a cleaning procedure if you do a search on the site in Google.