I have tried several things now to update to the latest version (0.981) but nothing seems to work for me. 

I have re-installed 0.75, regardless of whether I re-applied subsequent patches or not, the install of 0.981 fails - the other patches work fine. The drive or name of the install directory also do not seem to make a difference. And I have tried running the installs as administrator. 

I always get the same error. It does identify the location correctly but claims that it can't locate it - it doesn't make much sense to me. 

This is on a clean install of Windows 7 (64bit).