Today I got a new computer and installed foobar, copying over the config files and all that from my old computer to this one (via my external drive).
Try copying a few files to your new PC hard drive, just to make sure they play properly.
Run foobar2000 as Administrator (right click - run as Administrator) and try playing the files on the USB drive.
If that works check the permissions and ownership of the USB drive and its contents.
Everything is OK in Preferences > Playback > Output ?
(Or does it perchance try to play with the old device which isn't there anymore?)