You might get this message every time you try to install Silverlight 5, and there are no solutions offered. Your system meets all the stated requirements, you would definitely be baffled.
Your operating system does not support the 64-bit version of Silverlight.
Basically, the 64 bit Silverlight is what you would be given for download. You might need to check for the 32 bit installable version.
The 32 bit version could be downloaded here.
It’s always better to have the latest version for development purpose … go ahead make it work !