could be bad ram run memtest:
this version can be run in windows unzip the file to a folder then put the folder on the c: when you have done that go to the command prompt by going star>run and type cmd
once in the command prompt type cd c:\algermemtest\
then it should look like c:\algermemtest\
then type memtest /time 420
that will run memtest for 7 hours.
when finished or during it should tell you of any errors best left running over night.
The other problem it could be is the CPU, open up the case and check the CPU fan is spinning when the pc is on. Also go into the BIOS and check the CPU temps.
You can also download everest the program is in my sig, once installed open it and click computer then sensor and it will display your temps leave your PC on to see if it rises after a few hours and let us know what it says.
EDIT: also there might be more to the BSOD like a bad driver or something if it appears again try to see if anything shows up.