Simply put, here is what happened due to the above actions.
You replaced the Admins NTuser.dat with that of a user. The NTUSER.dat is the registry and ACL information for the account that that uses it. Doing what you did, was pretty stupid, but not dumb. You can fix it, but it's really not going to be worth the work. To really fix the problem, you will need to create another admin account. This should be possible even with the crippled account you now have. Once you do that, you will need to delete the DOcuments and settings directory for the Administrators account. This will force the account to be recreated when you log onto it next.
Windows is pretty smart that way, so unless you have another account named Administrator, you will be fine.
now, if you wanted, you could spend several hours fixing permissions and getting the admin account back to the admin level. This is very tedious and time consuming, but if you want, it's totally possible.
real_in,
Try last known good config, or if it doesnt work, then restore your system from the safe mode.
Just to let you know, Last known good is a useless tool of windows. It's designed to only function for device drivers and system configs. When looking in the registry, HKLM\SYSTEM\ there are several things here. CurrentControlset, Controlset001, Controlset002. Well, that is all that Last known good messes with. Problem is that once you see the XP banner, it's too late. XP and win2k take the currentcontrolset and write that out before they shutdown and so when the machine reboots, it's controlset001 is becomes the currentcontrolset, and then the last controlset001 becomes controlset002. This means that going to last known good, only works, if you know the machine with have a problem, cause once it's booted, you can't use it. Just wanted to clarify that for you...