Laptop keeps shutting down

Help - Im runnin a Toshiba L300 Laptop for 1.5 Years from now .i have upgraded it to Windows 7 Ultimate with 2.87 Gb Of RAM Intel (R) Pentium(R) Dual CPU T3400 @ 2.16 GHz 2.17 GHz . Now it just flashes a plain white or Blue Screen then i cant do anything i have to force shut down it to start all over again . when im doing something it kinda pisses me off . Pls Help me . BTW my laptop never heat that much
 
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