Ok. More information - All three are VMWare virtual machines so they have identical hardware. 1 works in XP SP2, 1 does not work in XP SP2, and 1 does not work in Vista SP1.
Yes, by 100% CPU usage the uosa.exe client is taking 100% of the CPU (in task manager) and not loading the enhanced client screen at all.
I did some research and back in July 2009 this was a problem that a patch to the enhanced client fixed, but that does not help as my client is 100% patched to the latest enhanced client version.
Ah well if they are all VMWare then that helps.
If I had to make a guess, some process is getting stuck trying to initialize (may be waiting on some system process that is stuck).
On the XP SP2 vm's, I am going to ask if they are identical and I expect you to say yes.
I will then ask, if by chance, is the non working VM a clone of the working one.
Are all three working on one real machine or are there multiple real machines involved.
If they are all on one Real Machine, have you ever tried running just one VM at a time? The point is, if for some configuration reason, VM's networking drivers may (not that I really believe this) be blocking or misdirecting the network traffic. If they are all on separate machines then this should not be the case.
I would expect all three of the VM's to be pretty much basic oob installations, with very little opportunities to cause this issue. Have you asked on VMWares forums what may be causing the issue?