Windows 7 & temporary files

Have recently installed GVIM 7.4 (2013, Aug 10) on a new Windows 7 laptop, and I'm having issues with anything temporary file related, and would really appreciate any suggestions please.

Current situation:
* Attempting to diff two files will always show them as identical, even when clearly different
* Shell commands that use temporary files (e.g. :'<,'>!sort or :read !date) report E483 Can't get temp file name
* Was seeing E810 & E97 until going through the below steps
I'm assuming the two are related, though I may be wrong. Apologies if that assumption confuses things.

I've done a little reading around, finding a fair few people discussing issues with diff and suggesting changes to the MyDiff function, but I don't think that would be related to E843, so have focused on the other suggestions, namely:
* setting my TMP & TEMP environment variables
* including let $TMP="c:\temp" & let $TEMP="c:\temp" in my _vimrc
* disabling UAC & rebooting
* setting broad access permissions on the Vim install folder
* changing the 'Start in' directory of the Vim shortcut icon (to My Documents)
* Running Vim as Administrator (though my user is in the local computer admin group)
* including set directory=.,$TMP,$TEMP in my _vimrc

Any further suggestions very gratefully received: I love Vim and use it pretty much constantly for a variety of little tasks, so it's frustrating that I can't work out what's different from my previous setup where these things worked!

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