There's not a lot of substitutes for Garageband given the price. Emagic was bought out by Apple several years back and Apple has used their technology to it's advantage. But since Garageband is basically a software synth and about 1,000 loops (much more with the expansion pack), you can get similar results using several Windows programs:
1. Band-in-a-Box is probably the cheapest...google "pgmusic"
2. Acid Loops is another good program very similar to Garageband with a better beatmapping tool.
right now 3. Reason by Propellerheads is about the best thing going and simple to use, unfortunately it's about $500 (a lot less if you are doing the bit torrent thing or .nfo files).
http://www.propellerheads.se/index.cfm
also 4. FLS Studio is easy to use and not that expensive check it out here:
http://flstudio.com/
also, Ableton is good but expensive as hell--like most of them. Band in a Box is your cheapest out there now. Oh, yeah, you might check out Reaper
http://www.reaper.fm/ wihich is Windows-based and only about $40 or so.
hope this helps.