I am not in a position to replace my operating system with a different one and the Unity download page stated that it supports Windows XP, Windows 7 and Windows 8 but (for some reason not specified) not Windows Vista.
UnityDownloadAssistant-5.2.0f3.exe will install on Vista- later versions require windows 7+
If you still have windows vista try downloading 5.1.2f1 version of unity.
i try UnityDownloadAssistant-5.3.0f4, it can install but when running, you will get error
“K32GetProcessMemoryInfo could not be located in the dynamic link library KERNEL32.dll”
cuz Windows Vista :\