It depends. Most machines that come with "recovery discs" will reject a standard Windows disc. Recovery discs are essentially a "disc image", which is a snapshot of what your drive "looked like" at the time of manufacture. Here again, machine and makers differ on exactly the "recovery option(s)" that will be available. Hopefully, your computer has a "non-destructive" restore option available, which is the equivalent of a "repair" installation of Windows. Some machines only have "destructive restore options available, which involves reformatting the HDD. You should avoid this at all costs, and certainly not use it before you have made every effort to back up your data.
It's always possible to install your hard drive in another machine, and copy your files that way. One thing, if you are running XP Pro and the HDD is installed into another machine running XP home, you will not be permitted to copy the files.
Constant reboot is often RAM. Have you removed you memory chips and run them one at a time? The obvious question here is does the machine contain more than one stick of RAM?