Is it just me? Or do the Athlon XP models have a locked Front Side Bus, and need pencil marks to unlock?
It is very dangerious though to overclock if you dont know what you are doing. Some motherboards for example, like ABIT usually have overclocking tools, and when you overclock too high, it resets the computer and restores back.
If your computer can't do this, it might be that your CPU is stuck at that rating and can't reset by itself if you did it within the bios
HOW TO FIX:
To fix this, either take out the bios round battery for a few hours or so (this will probably fix it since it'll reset it)
or if your motherboard has a CMOS MEMORY HEADER which is a jumper like on a hard drive (check the manual), you can move this to another position to reset it, and then just move it back after a minute or so. (Its in different places on every motherboard if you have one. it makes the job easier anyway, and resets it almost straight away)
This is what one looks like:
Hopefully you'd then turn on the PC to find everything booting up normally, since the bios would have held the FSB limits you'd set.
I hope works and so helps.