Windows 7 installed - used and shut down, now install?

Which edition of XP are you running there? Home or Pro? According to the system requirements XP Home isn't on the list of supported versions while XP Tablet PC is.

System Requirements

• Supported Operating Systems: Windows Server 2003, Standard Edition (32-bit x86); Windows Server 2003, Standard x64 Edition; Windows Vista Business; Windows Vista Business 64-bit edition; Windows Vista Enterprise; Windows Vista Enterprise 64-bit edition; Windows Vista Ultimate; Windows Vista Ultimate 64-bit edition; Windows XP Professional Edition ; Windows XP Professional x64 Edition ; Windows XP Tablet PC Edition

An x64-based or an x86-based computer with a 400 MHz or faster (1 GHz recommended) processor with L2 cache

Processor: AMD Athlon/Duron, Intel Celeron, Intel Pentium II, Intel Pentium III, Intel Pentium 4, Intel Core Duo, and Intel Core2 Duo

RAM: Add the RAM requirement for the host operating system that you will be using to the requirement for the guest operating system that you will be using. If you will be using multiple guest operating systems simultaneously, total the requirements for all the guest operating systems that you need to run simultaneously.

Available disk space: To determine the hard disk space required, add the requirement for each guest operating system that will be installed.

Virtual PC 2007 runs on: Windows Vista™ Business; Windows Vista™ Enterprise; Windows Vista™ Ultimate; Windows Server 2003, Standard Edition; Windows Server 2003, Standard x64 Edition; Windows XP Professional; Windows XP Professional x64 Edition; or Windows XP Tablet PC Edition http://www.microsoft.com/downloads/d...n#Requirements

Another item of concern is the hardware factor of whether or not your cpu supports virtualization. If not that would explain immediate problems. And by the way welcome to the sevenforums!

That shouldn't hurt anything since the memory requirement on XP is less then 512mb depending on how much you have loading in XP itself. You may want to disable a few items in the startup tab of the msconfig utility to make sure there's some extra just the same.

With a number of things loaded up on 7 and going to use the XP mode feature things can bog down a bit if 7 is loaded up with several gadgets at the moment or something else causing one or the other if not both to lag a bit.

(talk about lags it took over 8hrs. to see Vista installed on the Virtual PC on the 322bit RC there with updates! )

That often helps if you end up with a bad install at the start. Once you've been running 7 for a good period of time you'll likely be seeing another as well. Only when that time comes you'll be replacing XP!