Before proceeding with dual boot take your external harddrive and try to...

Before proceeding with dual boot take your external harddrive and try to understand partitioning scheme and partitions type.
Take a Live CD and boot your computer, any live cd is good for your needs, try to understand what kind of partition you've in front of you.
If your hd has ext3 you don't even need linux if you're happy with XP (personally i prefer linux but if you're happy with xp go with it), if you google around for a while you'll find ext3 driver support for xp and you can mount ext3 partition from your windows system, other partitions support may vary

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