Upgrading PC and OS but hardware still old?

Im getting a friend’s old PC windows 2000 but its a nice computer besides the OS. So I want to upgrade the OS and maybe change some simple parts of the machine but the motherboard will stay the same. Will my computer become faster and way better almost as good as a new one or am I just wasting my time? Only people that know stuff answer please and explain thanks.
PS: The windows upgrade will be to either windows 7 or Vista.

You will have to replace the processor if you want good performance. To do this you will have to buy a new motherboard to accommodate the processor. As processor series change so do the socket sets. The motherboard always has that certain socket on it.

Whenever I’ve built a PC the first thing I picked is the processor. Then I built around the processor. You want a good gaming PC? Then pick the processor first becasue all of the other parts have to work around that one part.

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