Open most programs gives me an error.

#1
I hooked another internal hard drive up to my computer. I already had two drives installed one SSD that has Windows 7 installed and one 1TB standard drive. I use the SD drive to install programs and keep files. After I installed the 3rd drive it changed my SD drive letter to letter E: and gave 3rd drive letter D: which is what the SD drive was before. Now when trying to do anything on the computer it pops up and says C:\whatever the specified path doesn't exist. When trying to do a diskpart from CMD I get the same error:
C:\Windows\System32\diskpart.exe
the specified program doesn't exist

Even though the programs is there.
When I try and download a program it does the samething.
I don't have a disk I can use to change the storage drive back to letter D:

I would have never thought this would happen since Windows is installed on letter C: and that is still there.
 


#2
I noticed I can open powershell up as long as I don't right click and goto run as administrator.
 


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