Sure, it can be done
Once you connect the drive to your computer, you should be able to see it in My Computer.
Once it's recognized there, the drive is part of your system. You can move files from your XP partition to that partition with ease.
You won't be able to move installed program or system files, but you've indicated that is not your intent. Just about everything else can be moved with no trouble.
You really don't want to mirror the C: drive, that is not necessary. When most persons speak of such, they do so in the context of backing up an entire system...which, IMO, is a good practice...but it's not the same as merely moving files.
You can also move your My Documents, your email store, all sorts of data files...from C: to the external.
Take a look at:http://www.techsupportalert.com/how_to_move_my_documents.htmhttp://www.insideoe.com/files/store.htm