Building almost new PC with old parts from my old computer?

You have the original manufacturers Windows XP? As long as you intend to only use it on one computer you can install it on your new one.

You will need the special set of letters and numbers to install it like the first time. You will have to re-register it again.

You can, as long as you don't use it with multiple computers using the same set up string of code to get it registered.

I tried moving an old hard drive to a new computer to save time and the new Motherboard did not recognize it because the old Motherboard had different settings and not to mention all the drivers were set up for the old hardware.

Deleting all the drivers and also, if you know how, all the old Motherboard's connectivity codes is a pain in the neck and can leave you with a bunch of real nasty ones after you got the old hard drive hooked into the new motherboard.

Recommendation: Copy all your good stuff off the old hard drives and re-format them. Buy an external HD case with a wired connector for them and use them as External USB HD's.

I'll be honest with you, once you buy the CPU and the memory, that is usually at least half the cost of the PC right there. And you want to buy the motherboard and the case, that is going to total around 75% of the cost then. Also, you will need a new power supply. Stock PSU's typically provide minimum power needs, and the newer electronics are hungry for more wattage, so you will absolutely need to ensure you have at least a 400-450 Watt PSU. You would have to check around, but I'm guessing that you have either a PCI, or at the most, and AGP video card. If it's AGP, you might not find a motherboard that accepts that, so you would have to add a video card to your list of items to purchase up front. That's basically the PC. You still need a hard drive and optical drives. And of course the monitor and peripherals. And an operating system. If you transfer your old hard drive, you can use the operating system that is on there....or should be able to anyways. But as you can see, building a system from scratch is pretty much an all or nothing affair. I would either keep using your computer as is, or buy another stock system from HP, Dell, or whatever. That would be cheaper than building your own.