Quoting Neil Banfield,
reply 1
It looks like the DWM is not enabled when you boot up.
Nope, I immediately checked Task Manager on boot, DWM.EXE is running Neil. Both on Windows Vista and 7. Strange thing is that I forgot to mention, it does not have this problem on every boot.
DWM.exe may be running, but it does not mean it is enabled.
You can tell by pressing Win key + tab. If you get flip3d then the DWM is enabled, otherwise it is not.
It sounds like a timing issue on your box. Do you still have TuneUp running?