The PC shutting down on its own are signs of your CPU overheating.
The things you'd need to mention -
1) Home built or shop assembled
2) system specs (details of your hardware including any aftermarket hardware)
3) How often this happens in a day
Shop Assembled. About 3 years old - received from the blind commission.
OS - XP Professional
V5.1.2600 service pack 3 build 2600 X86 based pc
processor x86 Family15 Model 4 Stepping 1 GenuineIntel 3000
Bios V1005.001 American Megatrends 9/7/2005 SIMBIOS v2.3
Hardware abstraction layer v"5.1.2600.5512 (xpsp.080413-2111}"
Tot Physical Memory 512.00 MB Avail 84.10 MB
Tot Virtual Memory 2.00 GB Avail 1.96 GB
Page fill space 1.22 GB
Drive C Size 149.04 GB Free 135.64 GB
If you need further info - please let me know - wasn't sure.