Spiceworks is free and works pretty well.
I use that for some clients, working on getting my linux monitoring box all setup using Nagios and Cacti.
If you just want monitoring, I think the linux is the way to go, cacti has a lot of capabilities for SNMP and graphing, Nagios is a good up/down/alert service.
But if you dont want linux to monitor windows, give spiceworks a shot. it will alert you of events, drive space depleting, etc......its an ad paid for service that you can pay like $10 a month to get with no adds though.