We use all dell hardware at our plant... We have a corporate license and corporate keys for all windows software BUT when we get new Dells in we just add the software that's needed and send them out the door...
So what we have is a Dell box with an OEM copy of windows running on it... When We virtualize the box it changes the Original Equipment Manufacturer ID to VMWare triggering windows to say "Hey I'm not on the correct hardware for this OEM license so you better register me!"
Well thats the answer to why the problem happens and here are two solutions to what you might do to fix the problem...
First try adding this field into your Virtual Machines config file (VMX File):
SMBIOS.reflectHost = TRUE
That will set the OEM ID from VMWare to what it cloned...
Second you can try to run a fix install of windows XP but I think this will only work if you have another Windows XP key laying around that is not an OEM Key...
I hope this helps you and anyone else in this problem! I looked high and low and only pieced this together from a couple of articles and posts... Here are the links to those for further reading...