Due to the fact that Windows 2000's Host-Resources MIB does not reliably provide memory usage, you must download and install the the no-cost 'Standard' version of SNMP-Informant, which can be downloaded from:
http://www.wtcs.org/informant/download.htm
I have attached the installer file in this post to make things easier.Just unzip and install informant-std-16.exe. The installation guide also attached, but not necessary. Just install this on the Windows 2000 Server , no other tweaking or configuration required. After installing scan for memory again on the ipmonitor.
This worked fine for me. Hope it can be helpful to others too.