Computer restarts in the middle while running an Application

My Computer restart quite often when i am running more than 4-5 Applications. i hav a AMD 2.4 Ghz Athlon Processor with 256 MB Ram 266 FSB no i hav removed the RAM and added a new RAM 512 MB RAM 400 FSB , The Clock Speed of the Processor is 266 MHZ,,

Please giv me a right rsolution to overcome this Problem.