Free monitoring tool for Windows servers?

I would like to try some free monitoring tool for windows servers.

I have 3 application servers with Windows OS and one database server with Linux. I also have one management server with windows and with connection to each of the servers.

I woul like to monitor use of CPU, memory, free disc space, if some process is running (windows/linux) and also windows event logs.

It would be perfect to have these information at one place from all servers.

I would also prefer ability to create custom "sensor/plugin" i.e. execute some script on remote servers and read its output.

I know about NAGIOS but I need something that can be installed on windows and also something easier to setup.

Can you give me some tips?