Press Ctrl+Shift+Esc and verify you don't have abunch of Javaw.exe processes running.
You can also go to Start> Control Panel> Java
Click the java tab and make sure the version of java you wish to use is the version enabled.
I also have the following config specified to dedicate memory to Java:
-XINCGC -XGX 6
In some cases, you may need to rename your .minecraft folder to old.minecraft and launch Minecraft again to recreate it. It sucks but at least you start fresh and and mods/plugins you were previously using you can easily copy them back.
