They can be "uncompressed" en masse from the Command Prompt. Start a
command session and navigate to the root of the drive and use the
Compact command, typically:
cd\
compact /u /i /a /s:\
That will uncompress all the files on the drive. For help on the
command use the /? switch, example:
compact /?
You can issue the command at the root of any folder, it will only
decompress the files in that folder and its subfolders. You can also
right click on the drive or any folder and in Properties>Advanced you
can uncheck the "Compress contents..."
John