Clientless transparent encryption

I am looking for a transparent encryption solution for a Windows based
server. I am looking for something that is solely server based and
does not require a client. This is because we want zero impact on
existing system which include some old clients such as DOS and OS/2.

I found one solution by Vormetric which fits the bill but it is very
expensive. I was hoping someone knew of another solution for this that
may be cheaper.

I need files to be automatically encrypted when saved to the Windows
share and automatically decrypted when requested. It needs to be able
to assign permissions based on Active Directory users and groups.

Thank you!
Karl
.