Software for Windows server monitoring [closed]

Question

Can anyone recommend monitoring software for a windows environment?

We already have monitoring in the sense of resources and availability. I want to know who logged in and changed settings, added files etc.

We have outsourced IT and I want to be able to track down all the changes to the system.

I'm open to keyboard logging or session recording.

Open Source/Free is optimal, but we're willing to pay for a solution as well.

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Edit:

It looks like we don't have the Systems Admin skillset yet.