You can slipsteam SP3 into an SP2 disc, and make it fully unattended using nLite. Using nLite, you can also save it as an ISO image, rather than burning it directly to a disc.
If you put a bootable ISO image onto a USB drive, then booted from USB though a BIOS boot menu...would that work?
Here's a link for nLite anyway:
http://www.nliteos.com/
P.s, They have another website, offering an updated programme to modify a Vista installation.
http://www.vlite.net/