XP files not being recognized

Help; I am running XP Pro and every few days I will get a message pop up (whether on or off line) saying "Files that are required for Win to run properly have been replace by unrecognized versions. To maintain system stability Win must restore the original versions of these files. Insert WIN XP Prof CD Rom" - More info on tab says "Possible reasons for this problem. You have inserted the wrong CD (NOT!!!) OR teh CD Rom drive in your system is not functioning."
I have no choice but to tick off that I have to keep the unrecognized file versions.
Why am I getting this error constantly every few days? I'm wondering if I need a new CD Rom? Is this possible or that what I have is not compatible.?
granny

either someone did some changes to your system or you have got hold of virus, run a virus sccan and see what happen

I know it is not a virus; have done several different virus checks. I am now wondering if the so called friend who installed XP for me did something to my system. He has since moved away. Darn, now I don't know what to do.
DO I buy new XP CD or new CD rom, or what.
?

If you have XP Pro disk. Reinstall you can do the recomended upgrade and it won't mess with your programs that you have installed only the OS.