If your power supply is making a fizzling noise, it could have faulty inversion circuitry (caused by age, overloading, extreme dust buildup), which could lead to load line noise and/or power brownouts to the system, both of which would cause erratic effects on memory, storage devices, and of course the CPU.
Or it could be just a noisy fan.
To determine if you have line noise you would need a very expensive oscilliscope. You could check RMS voltage with a regular multimeter.
My advice is to try a different, more powerful PSU.
If the same operating problems happen with a different PSU, you can probably conclude that it's software related, although it's still possible another piece of hardware is faulty. Cooling system, intermittent hardware connections, bad RAM (seen this a few times). Sometimes the BIOS POST test fails to identify faulty RAM, and you'll have to run a linux bootdisk with memcheck to thoroughly test the RAM.
Have you tried booting into safe mode? If you can make it into safe mode then do some virus scanning and some spyware/adware scanning, run hijackthis and look for shell redirects. Try a system restore.
Try going into BIOS and monitoring your temperatures, cooling fan RPMs and voltage levels to look for variances.
Good luck.
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Windows Vista: Just say no.
