I don't know what you mean by system. I have Windows 7 and I don't know what you mean by usage. The problem occurs randomly. Yesterday it didn't happen at all but today it happen 3 times in a 15 minute time period.
Your parts. Find "system" in the Control Panel, or use CPU-Z for a better view. The most important are the processor, power supply and motherboard models.
When does it happen relative to how long the computer's been on? Is it always a decent period of time after it's been started up?
Find a temperature monitoring application (Speedfan is usually good) and see what kind of temperatures you have at idle and under load.
Intel Core 2 Duo Cpu EZ email@example.com GHZ. Motherboard Gigabyte Technology Co., Ltd Model G31M-S2L. The power supply is Model ELT500AWT.
Temp 1: -55C Temp 2: -2C Temp 3: 31C HDO: 35C Core 0: 40C this one does go up and Core 1: 36C
Also sometimes it turns off when I first start it and sometimes it turns off after it has been on awhile. Hopefully the information I provided will be some help.