Quite frankly, as lazy as it sounds, in my house we pretty much just use normal email or MSN when we need to get something to each other. That being said, in answer to your question -
If Windows - I have previously used both HmailServer and Mail Enable Standard
I would highly recommend both, but if you want to use IMAP, then you really want to use HmailServer unless you wan to pay....
...Basically, I used Mail Enable Standard all the time but then I moved over to Hmail - Hmail has more features for free than the free edition of Mail Enable, however any of the pay editions of Mail Enable beat Hmail. That being said, you really don't need any premium features for your needs.
Just set up each pc the internal IP (or DNS) of the system running Hmail and away you go! You can have it set up in less than 2 minutes!
If using Linux / want to set up a VM, I cannot recommend and distribution over any others, but the easiest thing you can do is install Webmin and then you can install any of the many free mail programs (such as SendMail) and administer it all from a web GUI