Firstly any program you install on your main account will be available to ther users, unless explicitly chosen not to during the installation process.
You cant delete applications from the guest account as it's a limited user and doesnt have this ability...for security reasons.
In terms of missing space, try going into My Computer, Tools, Folder Options and then View...go down the list and place a tick in Show Hidden Files>
Now browse your entire C: drive, especially the Temp folder and Documents and Settings - you will probably find temp files that have not been removed OR some hidden files that should have been deleted.
I found 2GB of files in my sisters personal folder - they were hidden files and so not subject to the disc cleanup utility!!
If you have 2 versions of the same game, check in Add/Remove Programs - if it lists 2 delete one, if it only lists one then you may just have a second shortcut and not a second installed version. Right click each icon and check for the Source folder, they may be the same.
Just make sure that any files you delete you are certain you don't need!!!!