I can't help wondering if some of those bringing in encrypted files and hoping you could hack into them were really not the legitimate owners. Maybe they were trying to find out what their spouse or child or employee is hiding, or worse, maybe they just stole it and hope to get some personal info they can misuse for fun and profit.
Seems to me that if anyone encrypts a file, they'd better be sure not to loose/forget the decryption key or the program used to encrypt/decrypt the files.
There are several programs that can encrypt individual files or complete folders. Some archiving programs can also encrypt while compressing several files into a single archive file. It is possible to first compress a folder into a single archive file, then encrypt that archive file, of course, that means taking two steps - first decrypting then "unzipping" the archive in order to read it.
IIRC, Win XP Professional has a provision to encrypt files and folders, and I was considering an upgrade just for that feature. I'm not sure if that is included in any versions of Vista.