So, if you installed GE: S via installer, just go into your Steam folder.
There you should find the following folders "steamapps/sourcemods"
In this sourcemods folder there is your goldeneye mod folder as well as GoldenEye Source_Uninstall.exe file. Run it.
Second solution:
Hit your Windows key on your keyboard to bring up the Windows start menu. If you're running Windows 7 you can type the following in the search field, else use the run command from the menu (it's possible you have to enable it if isn't listed) and type it: "regedit" without " of course.
How to bring up the run command in the start menu if it isn't listed: right click on you taskbar and click properties. Chose the start menu tab and customize. Check the box for the run command to be shown, click ok or apply.
Once you're in the registry editor be careful to not touch anything (deleting/editing) of which you have no idea what it does.
Its layout is simple: just like the folder structure you're familiar with, you can navigate through it. Make your way to
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Windows\CurrentVersion\Uninstall\GoldenEye: Source.
In the right window you'll see the uninstall information Windows stores for this programm. Right click on the "UninstallString" and select Edit. Delete the ":" from the String and click OK. Now uninstalling via Installed Programmes should work for you. (This just tells Windows the path to the uninstaller file on your disk, if you moved you Steam folder since then be sure to enter the correct path then)