Windows 7 is intended as an upgrade to Vista; not XP. In fact you can't upgrade to Windows 7 from a Windows XP installation. I would suspect that's the overriding answer to your question.
I know, and that’s stupid. Especially when they put XP mode in the OS in the first place. They SHOULD have made it so if you installed 7 on a machine with XP it would put your existing XP in as the XP mode. The overriding answer is that they screwed up and missed a good opportunity to do something easy (given how close 7 is to Vista if Vista could upgrade from XP 7 could also if they’d bothered to port the code).
Actually, you can upgrade XP to Win 7. I posted a “how to” a few months ago. If you’d like I can find it, and repost, or pm it to you and you can post it.
You can also upgrade Win 7 beta and RC almost the same way.
Started work at 7 this morning and got the "Grey Screen of Death" with its Startup Repair program.
Every damn time an upgrade for ANY program calls for a restart this stupid crap pops up and disables my machine for 30-60 minutes, but not today. Not when I need to work from the get-go. No sir!
Today was a two hour FUBAR waiting for that crap to figure out it was an OS and not a permanent screensaver.