The root of the C: drive (and other select folders) are protected by Windows from being written to by misbehaving/malevolent programs. This is generally considered to be A Good Thing as it can protect you from a lot of grief if one's PC is ever compromised.
Turning off UAC and circumventing Windows other security protocols is not a good idea at all. If you should ever want to save something to one of these protected zones, just right-click on a program's shortcut & select Run As Administrator if it's just a one-time thing. If you need something more permanent, then you should go into the program shortcut's properties and select Run This Program As Administrator on its property tab.
The smartest method is to work with UAC rather than try to work against/around it. If you are trying to download something from your browser to the C: drive, make a folder, and save the item inside it. You won't be bothered by UAC at all then. The location C:\ is a protected space, but say....C:\Downloads or C:\Graphics is not.
Personally, I have my primary hard drive set up as two partitions. There's a small 75 GB partition & that's where I installed Windows. It's my C drive. I leave that for Windows to do what it will & for those brain-dead programs that don't know how to do anything but install to C: (Yes! They STILL exist!!). The rest of my primary drive is my D: drive. It's 1.8 TB or so & that's where I save all my downloads, install all my programs, and keep their data.
With this system even with having UAC on the highest, most secure setting I only see UAC prompts two or three times a week and that's when I'm doing something I know needs admin access like registry edits or installing certain programs that work at the system level. A sweet side-benefit to laying my hard drive this way is if the need or desire to reinstall Windows ever comes up I can just format that 75 GB partition immediately & reinstall Windows. I don't have to worry about copying/saving anything because everything is on the other partition.
(Oh, C:\Program Files and C:\Program Files (x86) are protected spaces as well. Using my system above everything is installed to D:\Program Files and D:\Program Files (x86) which allows older programs and games that don't know anything about UAC to work properly.)