First of all, you don't have to sacrifice anything, just copy the '.minecraft' folder on a backup usb or something and copy it back after the installation! To get to the .minecraft folder, in the start menu search: %appdata% and hit enter.
Second of all, I assume you already have a windows 7 product key ready. so, just download the win7 iso and follow these simple steps:
BEFORE YOU DO ANYTHING: Make sure you backup your drivers!!!
1) Get a usb stick ready that you don't mind getting formatted.
2) Download this program called Rufus, its for burning iso's onto usb's: Rufus - Create bootable USB drives the easy way
3) Open rufus and burn the iso
4) Shut down your computer and when it gets to the bios, click f12, then tell it to boot from your usb. if f12 doesn't do anythong, try f2, if that doesnt work search online for what keys you are supposed to press.
5) By this time you should have got to the windows installation screen. Now just set your language, keyboard format and your region.
6) Just follow the on-screen instructions.
7) If you did everything correctly, you have a nice, clean, fast os!
8) Now to get minecraft back, type %appdata% in the start menu again, and paste the .minecraft folder there again from your usb. Then, go to the minecraft website and download the launcher!