Do you receive any error messages or codes at any point?
What is the client executable you are using? JAMP or OpenJK (MBII Client)? You can check it in the Settings menu, the "Engine" option.
Do you see either jamp.exe or mbii.x86.exe in Task Manager processes list after starting the game? Does it just disappear then?
Do you have .NET Framework 3.5 installed? Try launching the launcher as administrator.