How to automatically run Selenium WebDriver test cases
CruiseControl is open source, and can do what TeamCity can do (just without the awesome interface, it may take some figuring out)
http://cruisecontrol.sourceforge.net/
MSTest command line options documentation live here:
http://msdn.microsoft.com/en-us/library/ms182489(v=vs.80).aspx
You can wrap it into a batch file and launch it using Windows Scheduled Tasks.
If you want to load all tests, it is pretty simply, just:
mstest.exe /testcontainer:Assembly.dll
Replacing Assembly.dll with the compiled test project assembly. The MSDN documentation should suffice. If not, feel free come back and ask
You can schedule to run your tests from a Continous Integration service like TeamCity.
Jenkins/Hudson is another great third party Open source Continous Integration tool with a neat interface.Pretty good community and plenty of plugins/add ons.
Extremely easy to setup and get going as well.More info here.