After downgrading from Vista to XP SP3 (5503) I quickly ran into problems, Windows Update keept failing to install 5 patches, it worked to start with but after Office 2007 was added things somehow got messed up and new patches failed.

Quite annoying because Windows XP kept informing me that these updates was ready to be installed.

Anyhow, the solution was found here;

Solution1 worked for me..
Stop the WU service (net stop wuauserv) and then run this command (regsvr32 %windir%system32wups2.dll), restart WU service and the patches installed perfectly.

Heard about this problem before, but never experienced it before now… perhaps this 5503 build of SP3 is not as final as promised 😉   oh well, could not wait any longer, Vista was simply not good for 4 year old pc.  And yes, the XP installation seem to run quite a lot faster, nice..