Regedit wont run, and grant admin full control context menu wont work

For some reason, my logon background image isnt showing up anymore, and when I try to change back to the way I had done it, regedit wont run, I've searched the forums for an answer, with no luck. And when I go to click grant admin full control, it brings up an open with menu, so I need a command script to grant admin to certain files again.

Windows 7 home premium 64 bit.

Please help, and thanks. And no, I dont have any viruses or malware, I take good care of my computer.

Unfortunetly I don't have a restore point, I made the changes to the registry more than a month ago, it just today reverted back to the default logon screen; and I would like to avoid adding software for something like this; I'd rather do it from the registry with a script; and I'd like to fix the registry so it'll open, I do still need it for other things. And I also need a script to add grant admin full control to my right context menu;

ok regedit is fixed, thank you very much, but do you know where a different admin script is, rather than take owner ship I'd like to Grant Admin Full control, for some reason the take owner ship script only works on a select few of my programs.

Ok now I cant open reg edit again, this time I'm getting an Error Message:

This Version of this file is not compatible with the version of windows you're running. Check your computers system information to see whether you need an x86 (32-bit) or x64 (64-bit) version of the program, and then contact the software publisher.

I'm running 64-bit.

It's where it should be, in C:\windows

The problem is, is that it somehow changed its compatibility from Windows 7 to Windows 95, it wont allow me to change it back to windows 7 because the software was made by them and came with Windows 7