My computer just restarts when it wants to. I can be doing anything on it and all of a sudden screen goes black and its booting up again. Can you help?

What type of PC do you have? How old is it?
With no info, I would have to say that it needs a new power supply or it could be overheating.

my pc is a pentium 3 450 hz , with 256mg of sdram, 32mg gforce 2 mx, 8.6 hdd, 32x cdrom and a 12x10x32a CDRW, its about 2 yrs old , the bery first pentium 3 what came out

Your system could be overheating. Remove the cover and turn the system on. Make sure that the fan on top of the CPU is running. Leave the cover off and see if your system is more stable. If it is, replace the fan.

This is most likely heat related. Either the CPU fan is not working or the power supply is not up to snuff. Without knowing about the PS in your system, its hard to be sure. In my experience when the CPU fan is not working, the CPU shuts off and the system does the same when it reaches a certain temp.

Try the suggestions above

herbie... If you have to leave your system set and cool off for awhile before It'll re-start, there's a heat problem. But if the PC is doing this shut-down and re-start in succession, then like the other guys have posted, go to the PSU. Luck.

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......Indecision may or may not be my problem......
...... Kickin' A Rock....