When trying to run the launcher it says 'could not find main class, program will now exist'

[hamus367]

Launcher Version: the latest one, the launcher build is 246

Operating System: Windows Vista

Java Version: not sure, i clicked the download link and downloaded it so i think its the latest one

Antivirus Program: Avast

Description of Problem:

when i download the launcher, i find it in my downloads an click it, it then comes up with the big box with run or cancel, i click run and then an error box pops up saying "Could not find main class, program will now exist", so i try running it again and i come across the same problem, i've tried uninstalling and redownloading but it still happens

Error Messages:

Error Log: