Install Ubuntu from USB disk

I am planning to install coming Ubuntu 9.04 on my Netbook, Acer one.
You know, Acer One netbook does not have CD-Rom. I don’t have USB CD-Rom either. So I have to use an USB disk to do this job.
First of all, I need to know the requirement of installation.
1) Can I boot from USB?
The answer is YES. I have an USB disk with bootable Ubuntu 6.10.
2) Have a harddrive.
Yes. Netbook has a SSD drive, 8 G.
I think it is big enough to install it along.
No dual-boot needed.

3) Can I rollback to Windows?
It is also very important for me.
I will do some research later.