Have you tried switching the method of connection when your phone is connected to pc? ie. activesync, storage card? also have you tried transferring directly to the card via card reader?.... Just some thoughts as to why they won't transfer. Possibly corrupted cab files themselves? Do you have backups of the cabs on your PC in a seperate location? I am thinking that somehow the files got corrupted. Is it every cab file and are they all in the same directory or spread throught the file system?

Just brainstorming some ideas.... Hope something helps! (I feel your pain!)