It's perfectly possible to install a second hdd and use that for your programs, pictures, movies, music, etc. You can easily run more than 1 hdd's in most computers as long as there's room for 2 in the computer and the IDE cable (thick gray cable inside the computer, assuming you don't have SATA since it's an older machine).
I, personally, have 2 hard drives installed. One for all my programs, and the other for all my music/videos. When you have it in a configuration like that, it's called "slaving" a hdd. The main (master) hdd is the one with the operating system on it, so that's the one that has priority over the slave (anything else). It's entirely possible to ONLY have the operating system on the master, while you install everything else to the second, slave drive. If that's your choice, just make sure to keep your programs consistant. If you start installing some programs to the master and others to the slave, the location of program files could get a little sketchy.
If you only want one installed, and you're going to replace the one you have now, there are multiple ways to transfer all the information from the old one to the new one, if you need to.
Edited by Heretic Monkey, 27 October 2007 - 08:56 PM.