Which version of Microsoft Security Essentials should I download?

I just got a new/refurbished HP desktop computer running Windows 7, so I need to download MSE again. There are 32 bit and 64 bit versions available, and I don’t know which one is right for this computer (it came with no manual). Can you tell me, or tell me how to figure it out myself?

Oh, also… I’m planning to do a restore from my external hard drive, to get everything from my old computer on to this one. Will that end up installing my old version of MSE, too? Should I hold off on downloading a new one?

PS: Thanks for helping me get this computer JohnPowell. You are an awesome friend!