In case anyone else is having the same problem . . .
I did the 5.5 to 5.6 upgrade on our SME server yesterday, and then applied the new virus-related blade (ours is a supported server).
Next thing you know, I had a number of users on Win 95 clients having trouble with e-mail and net browsing. Blue screens referencing fatal exceptions in IFSMGR(01), etc.
It appears that the issue is resolved by upgrading the tcp/ip stack to Winsock 2 on the Win 95 client (and then applying the accompanying y2k patch).
Both downloads can be found here:
http://www.microsoft.com/windows95/downloads/Robert