spontaneous blue screen/restart/failure cycle

Sometimes my Windows Vista (home premium) computer suddenly gives me the blue
screen and tells me that it is "dumping physical memory" (as well as to run
diagnostics). Then, it neither goes back to the normal mode nor restarts. So
I am forced to manually shut down the computer (by pushing the power button
for five seconds). Of course, I cannot run the diagnostics that the blue
screen has described because I cannot get the computer back to normal
(non-bluescreen) mode.

But when I manually restart the computer, it does one of the following two
things. Either A) it freezes, thus forcing me to manually restart, or B) it
starts working fine and then gives me the blue screen ten seconds into
operation.

When either of these things happen, I am of course forced to re-start the
computer manually, and thus repeat this cycle.

Fortunately, there is the Safe Mode (which I am currently using so as to
have Internet access). Obviously, permanently using the computer in Safe Mode
destroys the purpose of the operating system's bells and whistles.

Sometimes, the blue screen is followed by the computer spontaneously
restarting, then telling me that it was improperly shut down (and offering to
put me in safe mode).

So, can someone please help me get out of this nasty cycle?