I use PA.c as well without problems, however this is on vista and 7, systems without a floppy drive. The way I see it is that something to do with avast is accessing the floppy drive when you try to run one of the apps.
I can access of the file in my usb sticks without trouble
Does this mean you can open other files etc. besides the PA.c apps?
EDIT: a bit of a long shot, but this setting refers to floppy disks:
open avast! --> 'Real Time Shields' tab --> 'File System Shield' tab --> 'expert settings' --> 'Scan when attaching' tab --> UNcheck 'Scan diskette boot sectors...'
Like I said, long shot, but worth a try.