Longhorn 4074 fries my CPU :-(

Well, I got that fantastic Dell Inspiron 8600c (Pentium M 2GHz, 2 GB RAM, ATI Mobility Radeon 9600 Pro Turbo w/ 128MB video memory) which runs XP in Virtual PC faster than most of my other machines do on hardware, so I thought this would be the perfect machine for the WinHEC Longhorn build, incl. DWM and such. Imagine my surprise when (after the usual struggle with drivers) I noticed that it was slow and jerky. Disabled WinFS (the obvious suspect :-), no change. Taskmanager then revealed what was happening:


Whatever process I am launching, total CPU usage always sums up to >99%, there seems to be no idle task. The performance tab shows more information:


So most of the CPU cycles are used by the kernel. WTH is happening here? I'm clueless... Any ideas appreciated :-)

(Interestingly, one of my other machines also happens to be a 8600, with Pentium M 1.5GHz, and there, 4074 runs perfectly well. So I'm puzzled even more.)