Found it

C:\Windows\WindowsUpdate.log gave it away ...
2008-06-23 15:07:05:250 880 cdc Report REPORT EVENT: {180A3945-2C5D-4FFD-B363-404A5833B923} 2008-06-23 15:07:00:250+0200 1 184 101 {60B990A0-6EFA-47BE-8F5A-7DF2C402583E} 105 0 WindowsUpdate Success Content Install Installation successful and restart required for the following update: Windows XP Service Pack 3 (KB936929)
2008-06-23 15:11:45:984 2736 ab4 COMAPI Installation wizard (API) invoking RebootSystem
2008-06-23 15:11:46:422 2736 ab4 Misc WARNING: SUS Client is rebooting system.