Cleaning up an unknown Windows system requires three steps, performed in this order:
3. Performance tuning
This article discusses security and anonymization. Next month's article covers performance tuning. This article is based on my free guide How To Secure Windows and Your Privacy. The guide was published two years ago but is still relevant to cleaning up Windows. I'll leave out the screen illustrations in the guide, as well as its more detailed techniques.
We'll cover the highlights here. The goal is to answer this question -- how can you secure a Windows computer about which you can make no assumptions?
How to Secure Windows