Instructions in this post on Reddit.

There's also a thread here where one of our members has posted links to get the good firmware:
http://forum.chumby.com/viewtopic.php?id=9758

I'm not a windows person, but here's a windows md5sum generator I found that you can use to check that you've got a good copy of the file: http://www.winmd5.com/

The file you want has a md5sum of 35f49ae0b19aee592189297039cccd9e