how do i use xp disc to repair an o/s?

[bigmuddy11]

Hi, i'm better with hardware than software, how can i use an xp pro with sp/2 disk to repair an op system that won't start up, it has been playing up for a long time now,
no known viruses, no adware/spy ware no trogans, occasionally comes up with cpu settings have been changed? i just go back into bios and set back to failsafe defaults as it is not my computer that is all i was going to try so as to not do more damage.
have trierd the tools settings to make it scan and fix any file system errors and works for a while then plays up again, uncle is about to use it as an airplane and make it go for a flight out the window..... this is a fresh install of xp pro using a genuine disk only done about 4 months ago, i want to try using the disk to repair it to see if it will start up again as he had a "friend" have a look at it and i don't know what the friend did but after a couple days it simply won't even start up now. i am reasonably intermediate when it comes to computers and will try anything if it is explained carefully, thankyou.

[unreal]

I think if you boot the CD up, and go through the installation, it will detect Windows already installed as an operating system on there. Then it's all like "zomgwtf you have an O/S on here already, what you wanna do innit" and you can format, or repair the operating system. Select the repair console, it will take you to a command-based thing, and you can type /help or something for the command list. There's one there like /bsector or /boot which basically repairs the bootsector for the O/S... worked for me once.

Those are the steps of how I vaguely remember it, it may differ here and there o_o

Oh and this is for the hexus.help forum