You mean you've slaved (hooked up) your second hard drive into your current machine, so you can retreive some files?
The new OS in the new machine needs to take ownership of the files and folders on the drive.
In order to gain all read and write permissions for the new drive, you need to take ownership of the new hard drive using the Windows XP Access Control settings in Security properties of the new drive. To do this you need to boot into Safe Mode.
It's relatively easy if you follow instructions properly. Here are some guides to help you:
You must be logged into the Windows XP Administrator's account in order to modify access rights to files and folders. If you have Windows XP Home, this means you will need to boot into Safe Mode to because the Administrator's account isn't accessible outside of Safe Mode.