The hacksaw suggestion is good, but requires too much physical exertion. Tin hawkins came out with a song about an old computer which says "If I shoot it...no need to reboot it."
Sometimes rebuilding an old computer is more of a hassle and more of an expense than it is worth. The ability of the motherboard, the amount of RAM it can handle, the CPU speed will make the effort and funds you put into give very little return.
Building a new computer from scratch sounds like a huge project...but after you have read and studied up, and then sit down to do it...it really isn't that hard as long as you are slow and careful.
I believe in simplicity.
I start with a simple basic case design such as an mATX tower...

This is, for example is easy to work with, comes with a 480W power supply, rear case fan, power cord, and screw in stand-off mounts for a motherboard.
As far as mainboards, the simplest approach is to get a motherboard/CPU combo. Often times the CPU is already mounted onto the board, and a heat sink, CPU fan and cables are provided.

One of my favorites is a Biostar board with an AMD CPU. The board comes with a disc with the drivers, and it comes with installation instructions as well as a chart which identifies all the areas of the board and tells you what they do.

Take the model number of whatever mainboard you use and go to Crucial.com.
Use the model number to find the right type of memory for the board.
When you get the memory sticks in, mount them into the motherboard, then mount the board into the case. (making sure you have already installed the heat sink and CPU fan.)
Making sure you have a couple of SATA cables, get you a decent SATA hard drive and SATA DVDRW/CDRW drive. Plug one end of a SATA cable into the rear of each drive, and the other end into the SATA ports on the board. Connect the SATA power connections from the power supply into the back of each drive.
Connect the correct cables from the power supply to the mainboard. Connect the CPU fan to the mainboard. Connect the rear case fan to a power lead. Look at the chart/instructions of the board to guide you. Connect the case's USB cables to the board and the LED lights. (Instructions that came with the board will show you where it all hooks up.) Fortunately, power connectors are designed so that it's sort of hard to connect one to the wrong spot, but still, look closely and be careful.
Of course, with a new system you'll need an operating system. If you're going Windows go with XP or 7. If you can, get a 64 bit dual core CPU, get the maximum amount of RAM for the board, and a 64 bit version of Windows.