I had the same problem and had installed Thursby Systems application Dave trial version to test for a customer. I found after the trial period I was unable to access my Win Servers but Mac Servers were fine. Once Dave was un-installed they came back. This may also give you some hints as to where some issues may be on your side.You may want to look at some of the apps from Thursby as they are designed for bridging Mac to the Windows environment.
2008 MacPro 4tb, 9gb RAM OS 10.75.
Win. Servers work fine on all other Macs in office.
