Windows XP Boot up problems

I keep experiencing LONNNNG delays when booting up my computer. Although it eventually works, everytime i boot up it keeps stopping and rebooting half way through - it keeps returning to the page where you see windows xp on a black screen and the square colours flashing at the bottom to show that it is rebooting. You only have to tap the keyboard and it again freezes and jolts back to a reboot. Eventually it gets to the page where i can login as my user name ( the whole family have their own login names on this page) and it again goes back to a reboot. After several attempts - over an hour usually, it logs me in and i can start using my computer. It all seems to run ok, but I am starting to not switch it off because of this. Can you offer me any advice?

Thank you