I am experiencing a problem installing QGIS 2.4 Chugiak on Windows 8.1, resulting in a error loading Grass. This may be a problem only affecting certain installations, I'm not sure. I am a noob on QGIS and not so hot on IT.
The installer QGIS-OSGeo4W-2.4.0-1-Setup-x86_64.exe does not report any errors, but when QGIS is launched a dialogue box reports that there is an error in finding the Grass installation and asking for the folder where this is located. Pointing to the folder results in it being unrecognised by the dialogue box, which asks the same questions again.
This appears to have happened to others in the past (QGIS is not able to find GRASS. Choose the path of installation folder of GRASS) and occurs due to either of both of long path names not being recognised (or being correctly resolved to short names)and/or spaces in the file names.