The Video and Music Libraries can't be accessed if you previously used them with a path which no longer exists (eg. a mapped network drive that is no longer on the LAN, or a removable drive that was disconnected). As GB tries to reenter the last used path, it then gives an error that it's no longer valid (paraphrasing) and always kicks you back to the menu. Only fix is an app restart.
GBPVR System:
XP Pro SP2, AMD XP1800+, 512MB DDR, NVidia 6200 AGP (1.90.62 drivers), PVR-150 (2.0.43.24108 drivers), nVidia Purevideo Mpeg2 codec, PVRX2 1.4.7
XP Pro SP2, AMD XP1800+, 512MB DDR, NVidia 6200 AGP (1.90.62 drivers), PVR-150 (2.0.43.24108 drivers), nVidia Purevideo Mpeg2 codec, PVRX2 1.4.7