Computer Won't finish Load

My Grandsons computer get to the Windows page showing that it is loading but will not finish and go to start up. It just continues loading. :confused

Have Windows XP, AVG, SpyBot and Comodo Fire Wall.

Need advice as to where to start to try to fix this problem, any help would be appreciated. Wish he had GoBack but he doesn't!! That is what I use most times if my computer acts up, at least it gives me a place to start over and search for problems.

Thanks for your help here.