Try to make more free space on a system partition (in Windows usually C:). Unity uses system partition to unpack files, even if you install it on different partition. Unity need about 1.7 GB of free space for temporary files.
This is a temp file disk space problem. I just encountered it myself (different file, same error). I have all my system/browser temp and cache files on a 2GB RAM Drive - I checked the drive when I saw the error and it only had 12KB (yes KB) free - increasing my RAM Drive size to 4GB allowed the install to complete without errors.
Clear some space on whatever drive your OS temp directory is (usually C:, or the drive where Windows in installed).