Give this one a shot from your Nagios XI server's command line run the following, be sure to change the disk letter after PhysicalDisk(0 to one relevant to your server. Also, be sure to define the MaxWarn= and MaxCrit= values. Lets start with this disk check for an example and then work on finding one you need to use:
Once the Performance Monitor opens you will see a list on the left hand side, from here click the tab labeled Performance Monitor "Should have a graph icon next to it." Once this page opens click the green "+" sign at the top and search for Memory in the listing, then click the "+" sign next to memory to expand the list of performance counters, choose one from the list and let me know which it was. If you would like multiple counters monitored you will need to set up additional services.
Basically, my concern is to get Used & Actual memory.
I can see there are number of counters available under mem tab tried one of them, but no luck.
/check_nrpe -H 172.17.98.163 -c CheckCounter -a "Counter:=\\Memory Available MBytes" ShowAll MaxWarn=80 MaxCrit=90
CRIT: Counter not found: \Memory Available MBytes: Unable to parse the counter path. Check the format and syntax of the specified path. (C0000BC0)
The available memory shows exactly what you would see in the Windows Task Manager..
You could simply "for example" set your warning threshold to 1000, and critical to 500, this will let you know when your memory is being eaten up. Used memory out of total memory is not something windows monitors through it's internal performance counters, this would have to be done by some custom plugin magic and number crunching as it is not readily available. A great deal of plugins are based off performance counters and if the information never existed in the counters there would be no way to monitor it besides how I described above.