I doubt Windows is forgetting the Mac, but losing the connection. I would think if the networking was there and its configuration on the Mac didn't change, it would connect using stored information. Once the connection is lost, it will not reconnect without refreshing (F5). (I've not got 7 but I don't think a lot has changed under the hood except for Aero.) Therefore, I'm going to target the Mac as nothing changed on Windows.
Let's get this out of the way: I did not take this to be an issue with the Mac going to sleep but you might double check that's not the case. Having Filesharing on should prevent that automatically though it might be a bug if the Apple filesharing portion is off.
One simple solution is to create a new network location (Network>Location dropdown>Edit> + >«name it» then for the Airport section, create a manual IP address outside the range that the DHCP server within the router uses. Most routers addresses end in .1 and run their IP range starting at either 2 or 100 and go for 50 to 100 addresses. The wireless router's internal web pages will tell you what it's set up to use but you might be able to sleuth where the number starts based on what you see showing up automatically.
Try New Network Location
If you really think it's an issue related to the router change or every IP address is allocated to DHCP, you could nuke the config and start over. Create a new location as above but leave it with the defaults and see what that does. Doesn't matter what you name it while testing but if it does work, you'll want to delete the old Automatic location (presumed name) and rename the new location by double clicking on its name in the edit sheet.
Can you ping the Mac from Windows after the connection drops? What style of connection are you using, Map network drive with \\computername.local\share? Is there interference causing WiFi issues? Is the router faulty?