Computer automatically restarts

hi,

its been 2 years from now when the problem starts. i bought a secondhand computer then withn a month suddenly when i exit some programs like games. my computer automatically restarts. i thought it was a graphics card issue, so i bought a new graphics card compatible to my compter. but the restart thingy still happens. shat should i do? is it on my memory?

my specs are

intel pentium4 2.66ghz
512mb ram = 128mb+128mb+256mb-all pc2100
256mb 128bit inno3d geforce fx5500
80gb harddisk

help pls?
thx.....

There could be a few other possibilities.

• Incompatible drivers. (E.g. Graphics, sound etc.)
  
• Infected with virus/spyware or other malware.
  
• Overheating problem. your computer overheats and restarts.

Do you occur with any blue screen when the computer restarts?