Every so often a window pops up saying so windows files have been replaced by unrecognisable files then says insert the windows xp cd and nothing happens. Also I think this is related when I go on my computer a browse files you then go to help at the top and appeared I don’t now where from is ‘is this copy of windows illegal’ this then take you to a part of the Microsoft website.

You may have a virus or trojan or worm or adware or some other sort of malware. Your message is a little hard to understand. Run an AV scan, and download AdAware and Spybot, install them, update them, and run. Delete anything that looks suspicious. Some advise deleting everything because it's all reversible. Also get Spyblaster which keeps a list of bad stuff on your computer and blocks them from being installed (theoretically). Be sure your independent (not MS’s) firewall is enabled. Spybot AdAware

The 'is this copy of windows illegal' is a normal option for a windows xp user. As for the unrecognizable file error you recieve, Go to Start - Run, type: sfc /scannow Insert your windows xp cd install disc when prompted, that command will scan your system for any incorrect system files and replace them as needed.