Another experiment to try is to delete your preferences.text file that can be found in the following location, depending on what version of Windows you have.
c:\Documents and Settings<USERNAME>\Application Data\Arduino\preferences.txt (Windows XP)
c:\Users<USERNAME>\AppData\Roaming\Arduino\preferences.txt (Windows Vista\Windows 7)
There has been some indication that a corrupt preferences.txt file can prevent the IDE from launching.
If that does not work, from a command line, launch arduino.exe with the following command:
arduino.exe --l4j-debug
Then send us the launch4j.log file in the Arduino folder to see if that tells us anything.