By "client", do you mean
an agent or a manager? From context, it seems like you mean manager.
I assume you just want something that will read all of the SNMP data that the agent has to offer so that you know what to get with your specific program. How about
snmpwalk from the Net-SNMP suite? There are Win32 binaries, but they're not GUI.
GetIf is a Windows GUI designed for network-specific SNMP, but it contains a MIB browser. It works kinda okay from my recollection.
Neither of these support collecting traps, though. There's another program in Net-SNMP that will do that, but I forget which one right now. For a Windows GUI, I've never had to do that, but you can try
Trap Receiver.