Dear Lady, there are many possible causes for this behavior; here are a few things you can try to see if there is any improvement:

You may have a bad power supply -- test it if you know how or, if you have another one around you could put in just to check it out, you could do that, then run it and see what happens.

It could be overheating -- take the cover off and vacuum and/or blow off (with canned air) as much dirt as you can, especailly from the fans, power supply, and CPU/heatsink. Run it again and see what happens.

Have you tried running it in Safe Mode? If not, try that and see how it works. If it seems to run okay, then it is probably a software issue.

Bad RAM (memory) can also cause random shutdowns. It can be tested if the previous suggestions don't help.

Viruses and the like are also known to cause random shutdowns and/or restarts on occasion. If you can keep your computer running long enough, look through the posts in the Security section for ways to detect and remove this junk.

After you've tried some of these things, post your results. Good luck!

Note:Not Assumming you or anyone else is an amature !
1: Vacumning [static electrisity has over 20000 volts ,its only missing the Amps or is that Wats ]you computer is not for Amatures ,neither is testing a power suppy .
2: Boot to safe mode by hitting the F8 key on startup.

how long does it run for before shutting down?

Thanx for the great help everybody. When i opened the casing, there were dirts accumulating the ventilation. I've cleaned them and run my pc. Its ok now. I guess its overheated.

You'd be surprised how quickly dirt and dust can build up inside your computer's case- it isn't a bad idea to check on the accumulation every couple of months.