You haven't mentioned what version of windows servers you are trying to monitor or what services/applications you are interested in monitoring, but you are NOT limited to buying a commercial solutionor running a linux server yourself. You have several choices:
- Server 2008's Windows Performance and reliability monitor (included with server 2008)
which includes Windows Reliability and Performance Monitor, which is a Microsoft Management Console (MMC) snap-in that combines the functionality of previous stand-alone tools including Performance Logs and Alerts, Server Performance Advisor, and System Monitor. It provides a graphical interface for customizing Data Collector Sets and Event Trace Sessions.
It also includes Reliability Monitor, an MMC snap-in that tracks changes to the system and compares them to changes in system stability, providing a graphical view of their relationship.
Great technet article here, although it is not my favorite choice because of the Micro-softiness of it.
2 . Consider using a Software As A Service monitoring tool like the ones discussed in this post Any SaaS server monitoring tools around?