Hi,
I was running stellarium on Vista (SP1) and WinXP (SP3) at the same time on two computers and I noticed the 1 hour time difference between them although the clocks were set to the same time on both computers.
Eventually it turned out that Micro$oft did not include the WA daylight savings patch in their SP3, although it was released long before the SP3
The pre-SP3 patch does not work. The new fix is to download WindowsXP-KB942763-x86-ENU.exe patch.
Without the patch, clock may be set correctly but some software (like stellarium) will think that we are in the wrong time zone. Also updates from Intenet time servers will work now.