Another good way to back up/store/listen to mp3's is to get an iPod for Windows. Even an old 5 gig could easily hold your entire library. I suggest this rather than burning a CD or making your old HD a "slave". Some computers are not adapt at reading mp3 CD's well. And with changing out HD's, you always run the risk of messing something up.
Admittedly, an iPod is considerably more expensive than doing these things, but it has more advantages. You can take it with you, you can listen to the songs wherever you are, you will have the reliability of an Apple product, you can also use it as a home stereo, and there is no troubleshooting involved.
I beleive the iPod requirements for Windows are that you have USB 2.0 capabilities. Or perhaps you can use FireWire. But I think that is for Mac formatted iPods only.