For SQL Monitor 4.3 and later

The web server computer and base monitor computer (these might be the same computer) need .NET Framework 4. You can download it from the Microsoft Download Center.

For older versions of SQL Monitor

The web server computer and base monitor computer (these might be the same computer) require .NET Framework 3.5. You can download it from the Microsoft Download Center. 

If you have .NET 4 installed, you still need .NET 3.5 SP1.