Computer trouble

My desktop computer will not boot up. When I try to power the hard drive, the monitor says that the computer was not properly shutdown and gives me 4 options, start in normal mode, start in safe mode, safe mode with networking, and safe mode with command prompt. No matter which option I pick, the computer just keeps trying to re-boot itself and always comes back to this same screen. This computer is for my kids, they use it to play around. The operating system is XP, the processor is an Intel pentium 4HT. This thing is probably 7 years old, and probably just needs replaced, but I like to tinker with things. I'm by no means a computer expert, so any help to get me started would be appreciated. Thanks