Windows File Protection

Hi. I am pretty technologically illiterate. I know how to get the basics done, but not much else. I have Windows XP. I did a disk cleanup a few minutes ago, and a grey box popped up that says "Windows File Protection - Files that are required for Windows to run properly have been replaced by unrecognized versions. To maintain system stability, Windows must restore the original versions of these files. Insert your now." It doesn't say what to insert. If I click "more information" it says I've inserted the wrong disk, but I haven't inserted any disk, nor am I sure what disk I'm supposed to use. I've done a disk cleanup multiple times before, but I've never had this problem. Can anyone help me?

Insert your Operating System Installation Disk

Someone else set up my computer for me, and I have no idea if I even have that disk. I've looked for it and can't find it. Is there any other solution?