If you have an XP installation CD, you can pop that in and do a repair-install of XP and it'll fix Windows and keep your files.
But you probably won't have a CD, so you probably will have to go to a computer shop to get them to do it. I would try going to a locally owned place, because other places can charge a lot. I had a person that went to London Drugs to get that done, and they wanted to charge her like $200.
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Do it yourself:
If you want to try and do it yourself, you should start with trying to get a CD by either finding someone who has one, or downloading one off of the internet via torrent (this Lime-wirelly ilegal to download an XP CD, since Microsoft sells them for $3 online) like Limewire, uTorrent, etc if you've heard of those. Then using an ISO burning program to burn the files onto a blank CD+R or CD+RW (not CD-R).
Now you got yourself a CD, so now you can pop it in and restart your computer. When the blank screen comes along with the white text, see if it asks you to press any key to run CD. If it does, do so. If it does not, you need to go into your BIOS and fix that. If everything is working, just skip the paragraph below this.
Okay if it doesn't ask you to run the CD, restart again and keep pressing the Delete key repeatedly until you get into a blue type screen (not a bluescreen), this is your computer's BIOS, you can change things in here. You can move around with the arrow keys and enter, and other F keys as well to do things like exit and save. You need to find the boot-order for your Hard drive, CD-ROM drive, and usually a removable media or floppy. Once you've found those 3 things somewhere, you need to set it so the CD-ROM drive is the first one, and then the Hard Drive is the second, and floppy or removable media is third. Now click F10 or whatever you need to press to save, and restart and try to run the CD like in the paragraph above this.
Now after you press any key, it should take a bit and it'll boot up the CD and load. When it's done, you can tell it you are doing an Installation, and not going into the Repair-Console. The repair-console is totally different from a repair-install, so don't press R yet. Press Enter (I think that's right) for installation and then wait a bit for it to load the installation stuff. Now that it's done, it'll ask you if you want to do an Installation or a Repair-Install. You want to do a Repair-Install, so click the key for that. Now agree to the terms by clicking enter or whatever it is, and you should be loading a bit some more, then it'll start doing it's thing (repairing).
Now you should be able to see your mouse curser and stuff, and it should say how much time is remaining (approx). Just wait until it asks you for things. This part should be pretty easy. Just fill in some stuff and then finish the installation.
Man that is a lot of text