Hello and welcome to Seven Forums,
Quick question, why do you need to open notepad as an administrator? To solve your issue however,
1) Open C:\Windows and find notepad.exe. Can you run as an administrator from there?
2) Even if you can't, right click it and click Send To -> Desktop (Create Shortcut). Then right click that new shortcut and click the shortcut tab along the top. Click advanced and check the box that says "Run as administrator". OK all dialogue boxes. Can you run it as an administrator now?
Hope this helps