Don't expect that all your files will come back, especially after such a large install (WinXP). If you're lucky you'll get some of it back. Hopefully you didn't lose anything too important, like the Soviet-bunker-plans-you-need-or-else-the-KGB-will-chop-off-your-feet type thing.
Next time, you should back up as much as you can onto a floppy disk/CD-R/DVD-R/Tape/Ferrite Drum or whatever you use before you reformat.
