Created Issue: installer fails [10427]

ive downloaded the Silverlight_5_Toolkit_December_2011.msi and when i run it it gives an error:

[Window Title]
Windows Installer

[Main Instruction]
This installation package could not be opened. Contact the application vendor to verify that this is a valid Windows Installer package.

[OK]

any clues as to what the problem with the msi is?