Well, you could connect both computers to your home network and move the files between the two machines.. that's what I would do. Going to take awhile no matter how you do it. If you have less than 25,000 songs, don't mind paying $25/year to Apple, and the Windows box stays alive long enough, you might consider buying a one year subscription to iTunes Match.. that way your stuff is uploaded to iCloud and you can download it to the new Mac. There's an Apple support article here http://support.apple.com/kb/PH2718 about it and a Macworld review here http://www.macworld.com/article/1163658/itunes_match_what_you_need_to_know.html Match can't handle files over 200 MB in size, so if you have ripped any music albums into one big huge file, that might be a problem; most people don't do that, though.
Hope it helps.