No Startup Tone

Sir,
its happening more often, that my PC shut down automatically and then it don’t restarts..
and
also sometimes when i switch on my PC..i can’t here start up tone…and there is not dispaly on the monitor as well..
but after 8 or 9 tries..i start working as normal…and after two three hours.it shut down again in same manner..
i couldn’t fine out the problem..
also I have to slots of RAM 32 MB each..but the memory shown here after memory test is just 48 MB..
can anyone help me please…
Thanx in advance…
Tahir