Backup-enGB.MPQ/temp could not be created

Hello,
I have this issue when ive finished downloading, it wont start installingen because;

Patch could not be applied
The file "C:\Program Files\World of Warcraft\Data\enGB\backup-enGB.MPQ.temp" could not be created. If this problem persists, you may be able to solve it by uninstalling and then reinstalling the game. If you are unable to correct this problem, please contact Blizzard Technical Support. (InstallerFile::Create)

any ideas?
cheers