if you've copied the same files (that work) from your main machine, than it must be something related to settings. I can't think of anything (off my head) that could prevent the drive letter from being in the albumart label code.. (I'm assuming you do get the filename of the art as well as the path -- even if incomplete).. I'll check the code and see the possibilities.
EDIT: Please, post your 'musicpath' setting in RR.INI (the path of your music in RRConfig). Also if you can open winamp (without running RR), then saving the current playlist to an m3u file and post it, that may be helpful as well.