Well, folks we did it. Our upgrade from ESXi 5.0 to ESXi 5.5U2 has been running for a week. There was a little fumbling getting the VCenter Server and Appliance updated and running, and we have the expected VSphere Client on XP problem. We can connect to the host, though, so that is good enough for our purpose.
Also, for now our decprecated Adaptec 29320LPE and attached tape drive are working fine.
Thank you everyone for all your help and advice.
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David Chapman