You're going to want to go for one of the top two options, most likely. If you have a 32-bit version of Windows, choose the .exe or .msi of "7-Zip for 32-bit Windows." If you have a 64-bit version of Windows, choose the .msi of "7-Zip for 64-bit Windows x64 (Intel 64 or AMD64)."
To check which one you have, open the "System" window in Control Panel. It should be listed under system type.
One installed, simply right-click the .rar file and then choose "extract here." You can also choose to extract all of the contents into their own folder from the right-click menu as well.