> Next I'm going to test on 2.6.25-rc7-git5 with KVM-64.

Running on 2.6.25-rc7-git5 with KVM 64 for 48 hours now and no more hangs!
Note that the longest uptime of the Windows 2003 Std guest was around 20 hours before.
So it *seems* that 2.6.25 does indeed solve the problem, although a couple more days of testing is needed to be sure.