Motherboard and Cpu alone (or possible Ram as well) is not necessarily a good idea.
The worst part of this computer is probably the hard disk drive, especially if it's a 5400rpm.
So a limited upgrade would be to stay at P-Ata with the same mobo, processor, and a better disk, like a 7k160: I got one for 27 euro including shipment at eBay.de. BIG difference for limited money. Enough for Xp. I did it recently on a Tualatin, every user told spontaneously a difference. But it doesn't make a modern computer.
If changing the mobo, then make a bigger leap to something like an Athlon 64 and more, or rather a Core 2 duo, with Sata ports and Pci-E, and change the hard disk and graphics card as well.
In the first case, you can install Xp on the new disk, make the partition as on the old one, then delete all visible files and copy the old ones on the new disk using an Ubuntu Cd for instance, and it runs.
With a new mobo, it's far better to reinstall Win. Your licence number should be on the computer's case. If you received no installation Cd, then the computer must be able of burning one, and if not, borrow or copy one. Without a licence, buy a used one. Xp costs about 25 euro now.
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What are you planning to do with this computer?
For Internet, you need a secure OS, which means at least Xp (...W2k rather not).
For recent video games, you need hardware muscle.
But to type letters, keep all the hardware as it is, remove all antivirus, all Windows, and put Win Me +Office97 on it, and it will be faster than a modern combination.