Here's what worked for me.
1. Download the iso from Microsoft Technet
2. Use software (free) that allows you to mount an iso as a drive
3. Format your USB drive as NTFS
4. Open a command line, switch to the BOOT directory on the iso
5. Run bootsect /nt60 x: where x is your USB drive
6. Select everything (all files and folders) on the iso
7. Copy to the USB
Done. You have a bootable USB drive. EDIT
: Just did this procedure this morning and installed Win7 Home Premium on an A90 with a 32 gig SSD
. A full install, including Office 2007 Professional and Microsoft's AV product takes 12 gigs.