Yes you can pre-download it now and save it ready to install when you get your new pc.
Quote from: answers.microsoft.com
"How do I get MSE if I do not have an internet connection or have a slow connection? From a computer with internet access, you can go to www.microsoft.com/security_essentials. Once there, click on "locales and languages". There you will see the links for the 32 and 64-bit OS. When you select the one you need, just save the download file. To get the definition updates, go tohttps://www.microsoft.com/security/portal/Definitions/ADL.aspx and select the same for the definition files, and save them as well. While on this page, you can also view what changes have been made to the latest definitions."