Booting from a USB Portable HD

I need help,
I was trying to upgrade to windows 7,
but accidently deleted my entire C drive.
and windows xp was in there.

so my laptop wont even start now cuz theres no OS...
so it just says "missing OS"

now..
i have to just reinstall windows XP or windows 7...
but i cant figure out how.

cuz..

1.my cd drive in my laptop is actually broken..wont work.

2.i have a usb..but its only 2GB.



meaning i have one option left,
which is my portable hard drive (30gb).
i can connect it with a USB cord.

so is there any way i can boot off my portable hard drive thru USB.
and install windows xp or windows 7?

PLEASE HELP T-T!!!
thank you!