I only have a pc. I used to have a mac but after my house got burgled I didn't. But my pc has both Windows XP and Linux on it so I am open to a solution which uses either os. The files are on an external hard drive which is why I still have them
This will depend a bit on exactly what is in the files. They may be just audio, but they may have midi bits and pieces in there as well.
In your .band file, you should have a media subfolder, and the audio tracks should be in there (as .aiff files if I recall). Any audio app should let you import them and let you save as wav, mp3 or whatever. I'd use Audacity - full featured, free, multi platform...
If you have non-audio pieces in there, like midi data etc., your only option is to find a Mac and run Garageband on it - then you can export in whatever format you like.