Hard coded drive C is incorrect

[lextm]

https://github.com/MicksITBlogs/PowerShell/blob/master/InstallFonts.ps1#L51 and https://github.com/MicksITBlogs/PowerShell/blob/master/InstallFonts.ps1#L53 relies on hard coded drive C, but Windows can be installed to any drive. You should use windir environment variable instead.