Problem with SNMP Agent on Linux

I have a Linux client, which I am trying to monitor from a Windows Server 2003 machine. I have installed the AdventNet SNMP agent for Linux onto the client, and the CPU Utilization monitor is working fine. However, when I attempt to set-up the Memory Utilization monitor, I get the following...

The Monitor did not respond to the test request

Device Name host
Monitor Name MemoryUtilization
Protocol SNMP
OID .1.3.6.1.2.1.25.5.1.1.2
Possible reasons
1. Check if SNMP is enabled on this device.
2. The SNMP request might have timed out
3. The OID may not have been implemented in the MIB.
4. The device might be down

When using the MIB Browser, I can get loads of info...

hrSWRunPerfMem.1:552
hrSWRunPerfMem.2:0
hrSWRunPerfMem.3:0
hrSWRunPerfMem.4:0
hrSWRunPerfMem.5:0
hrSWRunPerfMem.6:0
hrSWRunPerfMem.7:0
hrSWRunPerfMem.8:0
hrSWRunPerfMem.9:0
hrSWRunPerfMem.36:0
hrSWRunPerfMem.37:0
hrSWRunPerfMem.38:0
hrSWRunPerfMem.57:0
hrSWRunPerfMem.58:0
hrSWRunPerfMem.59:0
hrSWRunPerfMem.203:0
hrSWRunPerfMem.324:0
hrSWRunPerfMem.351:0
hrSWRunPerfMem.363:0
hrSWRunPerfMem.1493:460
hrSWRunPerfMem.1673:0
hrSWRunPerfMem.1751:0
hrSWRunPerfMem.2402:592
hrSWRunPerfMem.2406:380
hrSWRunPerfMem.2417:296
hrSWRunPerfMem.2436:600
hrSWRunPerfMem.2456:728
hrSWRunPerfMem.2486:344
hrSWRunPerfMem.2567:436
hrSWRunPerfMem.2643:1136
hrSWRunPerfMem.2658:788
hrSWRunPerfMem.2678:2020
hrSWRunPerfMem.2686:1624
hrSWRunPerfMem.2697:984
hrSWRunPerfMem.2716:424
hrSWRunPerfMem.2726:968
hrSWRunPerfMem.2737:896
hrSWRunPerfMem.2748:4940
hrSWRunPerfMem.2857:1468900
hrSWRunPerfMem.2918:4872
hrSWRunPerfMem.2934:5568
hrSWRunPerfMem.2973:16280
hrSWRunPerfMem.2989:96
hrSWRunPerfMem.3006:412
hrSWRunPerfMem.3007:412
hrSWRunPerfMem.3008:412
hrSWRunPerfMem.3009:408
hrSWRunPerfMem.3011:408
hrSWRunPerfMem.3417:2332
hrSWRunPerfMem.3419:1448
hrSWRunPerfMem.3679:412
hrSWRunPerfMem.3907:1016
hrSWRunPerfMem.4112:1380
hrSWRunPerfMem.5063:1104612
hrSWRunPerfMem.5201:0
hrSWRunPerfMem.5304:0
hrSWRunPerfMem.5343:14896
hrSWRunPerfMem.5356:14888
hrSWRunPerfMem.6314:2056

...but the monitor is still not working. I have started the agent on the client to use port 161, but as the CPU monitor is working fine, I don't think this is a 'port' issue.

Any help would be greatly appreciated.

Thanks in advanced

Stephen