Replying to Formating computer harddrives

Anyone here done a clean sweep reformat on there computer hard drive before? my laptop is running really shitty latly so Im gonna buy a external hard drive, move all my inporttant files, and reformat my harddrive. Is it hard to do? or does win xp pretty much do everything for you except all your drivers? any help would be mucho apprecieated.

Any aside from my doucuments and mp3's, is there any files I sould move onto the external hdd?
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