With that age of PC, and the symptoms you describe, it's almost certainly the CMOS battery. (I imagine, btw, that it's resetting to 2002, not 2020.... :-) ) The CMOS battery is located on your motherboard - the main printed circuit board in your PC. You should be able to locate it fairly easily, as it's a silver coin shaped battery. Usually, they're pretty easy to replace. Have a look, take it out - the type of battery will be inscribed on it, and you can almost certainly pick up a replacement on eBay for virtually no money.
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