I asked that exact same question recently. I have a PIII 500 machine I wanted to upgrade without the expense. The short answer is you cant effectively.
After a LOT of research, the best answer would appear to be to buy a whole new base unit, or, if you have lots of new-ish stuff you've bought for your old machine, you can find some company to build you a new base unit using a new Motherboard, etc, and the newer parts of your existing machine. That could cost £200 or thereabouts, depending on the processor/RAM/etc you want.
Then again, if you buy a base unit for £500 or so, it's usually all tested, with warranty and often has XP or whatever thrown in.
kittykat, this really depends on what you are upgrading from. You tell us that your processor is 300MHz, but what kind of processor is it, what is your current power supply rating and what is your motherboard? Replacing the fan is not expensive and you will have to remove it to change the processor anyway.
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