boot the new system in safe mode, with your old drive attached as the secondary hdd (you want to boot the new copy of the OS.)
now, in my computer, open the old drive, right click on docs and settings. Now select properties, then select security. select advanced, then owner, and change the owner to the one the one thats on the primary drive. make sure to check the boxes at the bottom to do the subcontainers and such. click apply, wait a while (depending on how much data you have), then click ok, etc, etc until your back at the desktop.
That should do it for ya.
This is avail in both xp home, and pro.
will work for any folder (I use this technique at least 2-3 times a day at work to recover data for customers).
if you have vista installed as the new os, I believe it will ask you to take ownership if you try to access an protected folder. if so, just follow the prompts!
The error you get trying to boot the old drive on the new hardware is because the HDD controllers are different, and the right drivers are not loaded.