For anyone getting errors such as "Could not find the main class: Gui. Program will exit."
The fix to this is to open Control Panel, search in the bar "environment variables"
Click "Edit the system environmental variables" Make sure you're in the advanced tab.
Then click Environmental Variables.
In the user variables check if you have PATH, or CLASSPATH. If either of these have this address C:\Program Files (x86)\Java\jre6\bin delete them, if not, leave them alone.
Then go down to the System variables and check to see if PATH/CLASSPATH has this address C:\Program Files (x86)\Java\jre6\bin if so, delete, if not leave it.
On a side note, make sure you remember what you had in CLASSPATH/PATH as it is needed to play other servers. For some reason on Windows 7 the client will only run when you have deleted them. Hope this helps ^^
-Super