How to solve the "overscan" issue if the iMac is connected to an HDTV and using BootCamp and Windows 7?
Are you sure you need to have a software solution within Windows?  Many HDTVs let you adjust current display settings to disable the overscan.  See PCWorld - Tech Trend: For HDTV, Don't Overlook Overscan.  Quote:


  If you don't see every pixel you should see, you're not viewing a true
  1920 by 1080 progressive-scan picture. The name of the viewing mode
  differs from brand to brand, but it's commonly called Dot-by-Dot,
  Point-to-Point, Just, or simply Overscan Off.


To add to that, on my own HDTV, choosing "Studio" under the Aspect setting has the effect of eliminating the overscan.  I've also seen it called simply "PC" on another TV, and "Just Scan" on another.

Check your HDTV's settings ... chances are it has something like this and you aren't aware of it.  Though, admittedly there are some HDTVs without such a capability (sadly) and for these you'll need to find a solution that lets you reduce the resolution output to the HDTV.

Update: I found the following article explains well both the issue and troubleshooting steps: Windows Experts Community - Configure Your Display and Display Adapter.

Plus: some folks have successfully replaced the Apple video drivers in their Bootcamp Windows with ATI drivers, in order to access settings to compensate for the HDTV overscan.  See this blog post: Apple Macbook Pro (Early 2011) running Bootcamp Windows 7 with ATI / AMD Catalyst Control Center.  Got ATI video in your Mac?  Worth a shot perhaps, if you do.  Take a backup before messing with drivers, of course.