pc shuts down unexpectedly

First check the hardware to see if the power supply and cpu are operating properly.
The other option would be to check for any virus,also run scandisk to check the hard drive is not gooing out

we had a couple of pcs doing that.
for one pc, it was a hardware problem and we had to replace the m/b.
for the other pc, after finding no apparent h/w prob, we wiped the harddrive, fdisk, reformat, reload all the data, and it stopped.

As per the A+ Certification guide: "A computer that reboots occasionally probably has a faulty power supply".

I've seen that more than once myself too.

well,
I have come across the same problem over and over again in my business, as well. First off disable your cache on the motherboard in CMOS. Your system will run slow, but if windows still shuts down unexpectedly (and you are using a power strip-electricity can cause this too -if not try a different outlet) then your problem is either one of two things BAD RAM SLOT OR BAD RAM! IT IS THAT SIMPLE. GOOD LUCK!