annoying ass computer problem, please help

[geekdiggy]

have an emachines T1840 computer (yes i'm ghetto) and since november i've been having a shitty ass time with my computer. whenever i'm playing a large game, watching a large/long video, or scanning for viruses/spyware, the computer would just shut off completely and abruptly, as if someone hit the switch or pulled the plug out.

someone told me it could be a power supply problem in my hard drive and if that's so, is it because i'm doing things that take up a lot of power? is it actually a power supply problem? if so is there anything i can do to make my computer knock this shit off?

i'm not sure if it's a virus because the autoprotect on my anti virus software hasn't notified me of any virus related problems. other than that i have no idea what could be causing this problem so if anybody has even a clue what might be going on or how to fix the problem or need more details please PM me or AIM me with whatever help possible

help please thanks