I usually upgrade some of the parts in my fastest computer every few years. I would then transfer the old parts to my other computer. I would eventually upgrade my mother's computer using the parts from my second computer.
I decided to install Windows 7 on my mother's computer a few days before Microsoft stopped providing security updates for Windows XP. I found out that her motherboard didn't support the Windows 7 bootable DVD. The BIOS updates stopped before Windows 7 was released. A Google search of the chipset did provide a few different hacks to install Windows 7, but I didn't want to spend that much time and effort just trying to install Windows 7. I didn't want to upgrade my system yet so I decided to purchase parts for her computer. I bought an i5 processor, motherboard with onboard video, and 8GB of memory. I also ended up buying a SATA HD and DVD drive since the old IDE drives couldn't be connected to the new motherboard. After I bought these parts I realized that I also needed to upgrade the power supply since I couldn't connect the old one to the new motherboard. I had to replace every part in the case. It would have been cheaper to do the Windows 7 hack, but I prefer the perform improvements from the upgrades.