Unity player, will not download to play on face book ?

When trying to download current version of Unity on face book to play robot rising, after clicking finish on installed unity player applet, I get another applet stating to install manually. Which when I click on it to install manually it just repeats itself. So I click on error details and it says, This machine does not support SSE2 instructions. What does that mean and how do I fix? Also I had a previous version I was using on a face book game, but completely uninstalled it and installed the version that is on your website. I am using Windows XP Home, the current Fire Fox, Chrome browsers, same thing comes up. And according to your Home web page I am using (Unity plug- in version (4.0.0f7 and Unity engine version 3.5.6f4). Thank you

You need to install the Unity Webplayer plugin to play webplayer games, but you don’t need to install the Unity Editor.

I installed the unity player exactly from the spot your answer in blue italics took me. It does not give a choice, I hit download, everything is automatic from there to finish. Like I stated previously an error message comes up-"**

Blockquote

This machine does not support SSE2 instructions.*****