Warning - Read before installing
Sorry for the drama but if you are playing with a beta on a production machine, then isolate any other drives before installation. My PC has 2 removable disk trays and one internal data drive. I can't afford to have two good PCs (space is also an issue) just to do hobby testing, hence the trays.
First the good bits:
Win7 is great. You can read about that elsewhere. I loved how this version found three important drivers during installation, esp the Ethernet one (else it would not have seen the Internet, unlike version 7000 where I had to install the motherboard DVD to get LAN and Audio). It found my Brother printer driver and one more. (In a subsequent rebuild it found two different drivers, not the above three).
My CPU and BIOS support virtualisation and it was turned on in the BIOS, but I could not get XP Mode to work. Just an error with no explanation. XPM requires downloading Virtual PC and the packaged copy of XP (about 400 MB). XPM isn't for home users (apps that write to the hardware, etc) -- it is for running apps like the ones real estate agents, accountants etc use - something a contractor wrote ages ago and which they can't replace). It isn't for corporate deployments either.
The bad bits:
When I installed Win7 on the same drive as the 7000 version the second tray and the internal drive were connected. Installation took an eternity, as the process was checking all my drives for errors. It found and repaired one. I finished playing with Win7 and replaced the C drive containing XP Pro. This is where my drama started.
I was presented with a boot menu:
* Earlier OS
* Windows 7
* Windows 7
Huh? How did "it" know that I had previously used an earlier OS and another copy of Win7? This was a useless boot menu as the previous Win7 build had been blown away and XP Pro was on a separate tray. I recalled being able to edit the boot menu but didn't remember that it was a file called boot.ini for XP. For Vista/Win7 you use bcdedit.exe.
Anyway, I did a hasty search online and edited the boot menu by removing the two Win7 entries. Now neither XP or Win7 would boot. :(
[Long story cut short: I rebuilt my XP drive and all its apps, rebuilt the Win7 drive and rebuilt the data drive (thank heavens for a file-copy backup before touching a beta). I could not restore using Acronis True Image Home 9, as it said that my XP's /system32/hal.dll was corrupt - there's some web discussion of this.]
The right way for my situation was to disconnect all other drives when installing XP Pro or Windows 7. Now both those OSs are happy to work in my PC when I swap the trays, even with my other drives present.