Subversion and micro-ISVs: This is a good little article on Subversion and how wicked good it is. We use it.
The solution is Subversion. I promise you that Subversion will change your life. It is free, open source, has a wonderful Windows client called TortoiseSVN, and uses unique numbers, incremented one by one, just like Perforce, for every change.
In fact, it goes further than Perforce and stores every state of the file system — that is, not just changes to files, but file moves and renames. You can give arbitrary attributes to files and it stores and tracks them too.
This also happens to be the first post to a new blog by the guy who wrote the newly released book “Micro-ISV: From Vision to Reality,” which I just got in the mail yesterday.