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It probably should run on windows though its not a problem if it doesnt, free would be good if possible but not essential.
I have no experience at all with anything like this so i'm a bit lost.
All of them will record customer details, and associate contracts to the customer. Any of them can do all of the things you mentioned, I think, (most of my experience is a bit further upmarket). I'm also pretty sure all of them have a Windows flavour. If you do find something free though, let us know