I'm running TortoiseSVN on Windows 7 64-bit and I kept getting an error and fail message "svn can't move \my directory structure\tmp\entries". Then it would say the file or directory was corrupted and unreadable. However I was able to get around this problem a number of times by doing another SVN update.
It turns out there is a problem with Windows 7 64-bit, TortoiseSVN and anti-virus scanners. If you shut your anti-virus scanner off everything works just fine.
In my case I'm running Microsoft Security Essentials (MSE), which of course doesn't have a shutdown option. To shutdown MSE you have to go into Windows Services and select "Stop" to manually shutdown Microsoft Antimalware Service.