|Could be a number of things but you will have to be more specific with info. |
1. How old is the computer and has it had a spring cleaning in recent time? Dust can be the devil to older computers, clogging up fans, heatsinks, etc. This can lead to overheating which can cause shutdowns.
2. What specifically are you doing at the time the system shuts down?
3. Has it just recently started having the problem?...and how much time between one incident and another?
4. Did you recently install new hardware? Usually on pre-fab systems like Dells and such, installing extra hardware can sometimes overtax the PSU which can in turn cause your problem.
5. Sometimes program conflicts can cause crashes.