Xp pro (not authenticated) how do I remedy ?

Hello,
My computer came with xp pro sp2 already downloaded but without registration sticker or disc. It keeps coming up with "you may be a victim et al" every time I start it and also the error "some of the files are missing, redownload from disc" Is there a way that I can register it without having to pay full retail for a copy of the program to microsoft. I purchased the computer from a tech who builds systems but it was a fair while ago and I have lost contact with him so cannot get the number of the certificate of authenticity from him. I am an invalid pensioner and have limited funds to purchase a new program. Can I get a cheaper OEM version? If so how do I download it without losing all my files etc on my computer? Thanks for any help you can give me. babe899