We are using all 32-bit editions of Windows XP pro, and 32-bit editions for Windows Server 2003. Definitely, not all machines are involved in this problem. I notice that machines with more software installed have higher degree of missing items. For example on machines with less that 20 installed applications (3 machines checked) we have no problem detected, but if we have more than 50 or 60 installations (3 machine checked), big amount of items is missing (20-30%). It is not application orientated problem, for instance WinRar is detected on one machine, but on other is not. Also, there are items in software tab that are not listed in add/remove programs (HotFixes), but that is not problem at all.
Regards.