xp pro intel pent.4 D

my xp pro came up with a screen saying that I needed to put original cd in because system 32 was corrupt or lost after running registry cleaner and rebooting it cleaned it allright and good.My mom got me this pc a while back she bought it from a retailer and they didnt have any paper work on it at all no cds nothing not even a registy key sticker I mean its cleaned off,I got mom to call and see if they had a cd for it and they told her if I wanted one to get it from dell.So I went on ebay and ordered a new Dell windows xp pro cd,I got it and it is a Reinstallation Cd for microsoft windows xp pro says only use this cd to reinstall the operating system on a dell computer this cd is not a reinstallation of programs and drivers tyhe date on it is1996/2001/ lmao probly bout when this on e was made anyway now I dont know what to do. I havent even opened yet because I been reading about it since it died on me and the reg key seems to be a factor plus I didnt know what steps needed to be taken to do this .Should I have got the cd with reg key? I been reading that you can find reg key after you get it running by links.This is a older computer to it looks like it any way the graphis card is a matrox and I have had a few tell me they were crappy lol, I cant tell all info on it cause I dont know it actually I knew something was gonna happen to it so I was trying to make back ups for it ,it came with the nero on it I had never used it b4. I do have a back up of what I think is just my docs when I tried to do a full backup it kept saying that programns were running and that it couldnt verify if it backup would work .I looked I didnt have anything open anyway I could use some input if anyone could help Thx I have never done anything like this b4 when I hear bios it scares me lmao seriously so I need to know every step I need to take I am not even sure If a dell cd will work is there assingned cds to certain computers ok gonna stop b4 I write another novel ok readding it has a ASUS-A8V-X mother board ,HDDBarracudda7200.7120 gb,card Matrox G45MT07580,im leary of this cd now because i didnt see no where it being a dell,it was a custom built pcsold to a store who sold it to mom,plusthere isnt a reg key on my pc,oh the seller of the cd i boutgh replyed said he would sell me a reg key dor 20 bucks ,i dont know what to do