PC Keeps rebooting?

Every time I switch the computer on it trys to load windows xp but just before the end of the installation it reboots and this is a continual cycle. I have tried f8 and getting it to go into safe mode but it will not let me do this. I inserted my xp installation disk and managed to get a c: prompt. After typing in chkdsk /p it tell me I have one or more errors in the volume.Please can you help I do not want to reformat the hard disk as I have lots of important information and family pictures which I could lose.