It's no secret, Windows 7 is the best windows ever--it doesn't matter that the RC build 7100 not the final version, it's still a LOT better and faster than the latest and most optimized Windows Vista.
OK... anyhow... some people don't have CD/DVD drives in their PCs, so using a thumb drive is usually the best way to go.
In Windows Vista
Start --> type 'cmd' --> right click on cmd and select "Run as administrator"
Select Disk 1 (Replace 1 with number reflecting your USB Drive)
create partition primary
Now that the thumb drive is done, extract all the files from the Windows 7 iso with 7zip
now reboot and go into your bios, and select your USB drive to boot first... or if your bios has boot menu selection, then use that and select your USB drive. (e.g. the boot menu can be accessed by hitting ESC for my PC)
And as always... to have the most hassle/trouble free installation, please do a CLEAN INSTALL.
Meaning: do no upgrade your old windows. At the Windows 7 Installation, do the custom/advanced install and format your HD that you're going to install windows 7 on.