Simple (freeware or open source) server monitor service

I'm looking for a piece of software which I can install on my client's Windows based servers which does the following:

1) Runs as a Windows Service
2) Monitors basic system health - CPU usage, Hard disk space, memory usage, the running state of services or tasks running on the server, RAID array health etc.
3) Monitors typical online services - HTTP, FTP, VPN, Ping tests etc.
4) Reports anomalies by sending an email

I'm looking for something fairly small, lightweight and simple, and hopefully free or open source, which I can install on all my client servers to warn me of servers that are about to run out of resources, or which are having service problems.
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mcsweenSr. Network AdministratorCommented:

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For a small environment, you could use something basic like Paessler:
However, it would be better to buy in something like Solarwinds Orion NPM, which is inexpensive, easy to set up, and is very customisable.

Try server alive its free up to 10 server.
Frosty555Author Commented:
All good suggestions. Points distributed based on what I can see supports my needs the best
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