Hmmm - I just did this from my old Vista x64 partition to my new Vista x64 partition - and had no problems (copying info that is).
However, trying to boot your old XP partition in your new comp is not going to work - the mobo, and other hardware, drivers
are just going to be too different for XP to be able to cope.
Are you running Windows Explorer with the "Run as Administrator" enabled (find windows explorer in start and right click, then select "Run as Administrator" - because you will need full admin privileges to do so).
If that does not work, then here is an idea - on your new computer, try creating a new user - that has the exact same username and password as the user on the XP partition (and make sure it is not a limited user - IOW, it is an administrator
account). Now change (not switch users but actually log out of your current user and log into the new user) and see if you can copy the info. If you can, then you'll want to copy every thing to your 'public' folders (create folders in C:\Users\Public) so that you can boot back into your other account and still get access to them.