See this thread in another forum, sounds like this is probably caused by a corrupted
registry,
http://www.techspot.com/vb/all/windows/t-12012-admin-icon-missing-on-welcome-screenxp.html
The guy did a parallel install of XP to his C: drive (C:\WindowsXP instread of C:\Windows)
then copied over Documents and Settings from old to new. Had to reinstall all apps
but at least he was able top save his personal data.
Yet another example of why a good backup strategy is the ONLY way to handle these
kinds of problems.