The difficult part with managing SNMP via Group Policy is that SNMP is not installed by default. The first step is to install SNMP on all the machines you want to monitor via SNMP. This can be managed a couple ways. The simplest method that I have used is the one Zenoss recommends. If you only have a couple of machines to install SNMP on, it may be easier just to go into the Add/Remove Programs --> Add/Remove Windows Components --> Management and Monitoring Tools --> Simple Network Monitoring Protocol.
Once SNMP is setup, you need to create a custom ADM template to manage the SNMP settings via Group Policy. This sounds more difficult that it really is. The bonus is that once you see it done a time or two, you really start to understand the power in home made ADM templates. I would checkout the mailing list at ActiveDir.org and the information available on petri.co.il. Both of these resources are invaluable for getting things done in Windows and Active Directory.
If you just want to manually control the SNMP settings, just go: Start --> Run --> Services.msc. Select the SNMP service, right click and select Properties and click on the security tab. All your communities can be managed through that tab.
Unfortunately, MS didn’t make SNMP as easy to implement in Windows as *nix environments have. However, setting it up on Windows is a good experience and all the work will pay off in spades once it is complete.
As for monitoring packages, check out Zenoss and Zabbix. I’m all for the open source, service support model of software development and these two have proven it is a viable means of running a productive and profitable software development business.