Data Execution Prevention (DEP) on Windows Vista and above. This does not occur on every machine, but some users have reported DEP can stop Unity from working. To add an exception follow this guideline : Windows help & learning, “To turn off DEP for an individual program”. Note: It is reported (but not confirmed) that Oct '11 security updates to Windows change the DEP setting to highest and this can stop Unity in it’s tracks. Change back to the settings mentioned earlier.