I use WinXP + cygwin's ruby and have no problems to use the latest backgroundrb.
I have used BackgrounDrb on my XP for some heavy tasks and I do not encounter any problem with it. The only problem that I have with it is when I ported my solution to use SQLite with concurrent approaches to the embedded data base, namely resource locking. But, that's the SQLite problem not the BackgrounDrb's.