It sounds as if your CPU fan might be going.  Inexpensive to replace.  I
would try that first.  Power supplies are usually very durable,
particularly if kept clean - no 'dust bunnies'.

> Hello.  My computer is a Dell Dimension 8100, P4, 1.4Ghz, 384mb mem,
>  40Gb HD.  I have virtually let this computer run non-stop since 2001,

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