A bootable live Windows XP/Server 2003 environment. See http://www.nu2.nu/pebuilder/.
Also its bootdrv.com is one of the smallest useful programs I've ever seen, clocking in at seven bytes.
I still remember the first game that was soooo big it required both A and B, meaning you to take the OS boot disk out (which you of course always forgot to put back in after you were done.)
Good thing I had a dad who soldered Philips' Apple ][ clone together when I was 2 so that I don't feel that old for knowing this ;)