When i try to download Arduino IDE i get this error message:
"Installer integrity check has failed. Common causes include incomplete download and damaged media. Contact the installer's author to obtain a new copy.
More information at: http://nsis.net/nsis_error"
PS: i'm on Windows 8
I've loaded both 1.0.6 and 1.5.8 on Win 8 64 bit. Erase the source files and re-download.
i re-downloaded it around 20 times but it doesn't work 
Experienced the same problem & performed the recommended steps including adding /NCRC to the cmd line. All to no avail. Eventually gave up trying to download 1.0.6 from the Arduino site after many attempts. The problem I think is always with a faulty download, not with the installer (hence NSIS is behaving as it should).
Accessed an older version (1.0.5 r2) from Google Code Archive - Long-term storage for Google Code Project Hosting. and although slow to download, it installed first time. ;D
Running W 8.1 pro