here is what you need:
1) win7x64 .iso (you can download it
legally here: http://www.wincert.net/forum/index.php?/topic/8906-official-windows-7-sp1-disks/
2) mount the .iso file on a virtual cd/dvd drive; if you don't have anything like that, you can also use winrar to extract the iso
3) copy the files that are inside the iso image to your usb (the files take ~3,1gb)
4) once everything's on usb (let's say it's X

, open command prompt (winkey+r, type in
cmd press enter), type
x:,
cd boot, once you're located inside x:\boot, type
bootsect /nt60 x: and hit enter, you should get the following message:
Target volumes will be updated with BOOTMGR compatible bootcode.
x: (\\?\Volume{########-####-####-####-############})
Sucessfully updated NTFS filesystem bootcode.
Bootcode was successfully updated on all targeted volumes.
incase it says 'access denied' then try
bootsect /nt60 x: /force
congrats, your usb now contains bootable win7x64sp1 installation
one thing to note is some systems don't support booting from usb if:
a) your flash stick is not plugged exclusively straight at the back panel of your motherboard
and/or
b) you specifically set usb stick to be the 1st drive in boot order (some boards won't boot if you just pick to boot from stick via boot menu by pressing esc/f11/or whatever the boot menu key is for your motherboard)