Here is what I did for people who have a netbook but found in hard doing it with a bootable usb:
1. Buy an external enclosed dvd drive unit, they sell for around $30.
2. Setup BIOS to boot from CD/usb unit.
3. Put the operation disc in the DVD unit and install the same way as if your netbook had a CD/DVD drive.
4. This is the best way for people who are not that techie, and also this way saves you time but not money, which is sometimes a good trade off.
*But the way I did mine by using an old usb hard drive enclosure. I have a desktop computer and this is what I did:
1. Removed the backing on the enclosure unit
2. Open the desktop computer and removed the ide and power cables from the dvd rom drive.
3. Installed the enclosure backing on the dvd drive
4. Hooked up the usb connection from the unit to the netbook.
5. Setup the Bios to boot from cd/dvd drive
6. Installed windows xp