Setup.exe fails, folder " "does not contain a valid media info file 'mediainfo.xml'"

I'm trying to run the SQL Server R2 setup.exe, and all I get is a command window popping up briefly, then nothing.
This actually affects all SQL Installers that I have on my machine:
2008R2
2012
SSDT BI Pack for VS2012

If I run the setup.exe from the command line, I get this:

An error occurred during the SQL Server 2008 R2 Setup operation

Error result: -2067529698
Result facility code: 1220
Result error code: 30

For more information, review SQL Server R2 Setup Logs in your temp folder

The contents of that log file are:

04/04/2014 10:54:34.489 ======================================================================
04/04/2014 10:54:34.510 Setup launched
04/04/2014 10:54:34.541 Attempting to determine media source
04/04/2014 10:54:34.558 Media source value not specified on command line argument.
04/04/2014 10:54:34.574 Setup is launched from media directly so default the value to the current folder.
04/04/2014 10:54:34.591 Media source: C:\SQL Server Installers\2008 R2\
04/04/2014 10:54:34.608 Attempt to determine media layout based on file 'C:\SQL Server Installers\2008 R2\mediainfo.xml'.
04/04/2014 10:54:34.627 The folder 'C:\SQL Server Installers\2008 R2\' does not contain a valid media info file 'mediainfo.xml'.
04/04/2014 10:54:34.649 Setup closed with exit code: 0x84C4001E
04/04/2014 10:54:34.677 ======================================================================

I've checked 1000 times that the folder DOES contain a mediainfo.xml file:

<?xml version="1.0" encoding="UTF-8"?><MediaInfo><Properties><Property Id="MediaLayout" Value="Full" /></Properties></MediaInfo>

I've seen a few suggestions elsewhere in search results, but nothing has worked so far.

I've previously run the 2008R2 installer and the 2012 installer without problems, so I'm not sure what has changed, or how to get round it.

I'm running on Windows 7 Enterprise SP1, 64 bit with up-to-date Windows Updates.

Any help would be very much appreciated.