Hi Gordon

Had a look at the spec. of this rig and wondered what Power Supply Unit was installed. The specs. said 375W, but if you are running a NVIDIA 8800GTX card that seems barely enough.
Check again AT CONTROL PANEL<PERSONALIZATION<WINDOWS COLOUR & APPEARANCE...Ensure "Enable transparency" is ticked. Set the colour intensity to far right and see if their is a difference.
If you cannot get the "aero" effect it could be the PSU.
Also check that you have the most recent drivers installed for your graphics card.
Visit NVIDIA site and look for most recent drivers (version 169.25 or Beta 174.74)
Download and follow installation instructions