I have heard of a number of things required to fix this, mainly involving ensuring you do not have conflicting adb.exe files linked in your PATH variable.
The issue with mine is that I am using Genymotion and it was not configured to use the SDK previously installed on my machine. How to fix this can be seen here.
If you are using HTC Sync or another custom backup application that may contain adb.exe, you can try this tool: ADBFix
Once you have made your changes kill the process running port 5037 as specified by Dan. For me the process was AndroidInterface.exe. Then navigate to the platform-tools folder of the SDK and run adb start-server to restart adb.