why my comp always restart.pls help!?

After i formatted my computer on october’08 due to re-starting problem, it was running finely for a few months [2-3], then the re-starting problem start again (auto-restart). Once i open my computer, it would restart several times and most of the times restart before even it reach the welcome screen.

My computer is Microsoft Windows XP, Home Edition,version 2002, sp3.

Last warning it promt was
“window could not start because the following file is mising or corrupt: system32\DRIVERS\isapnp.sys”.

Can any one PLEASE help me over here??
pls!!!