yes. you can download ISO and burn it dvd. Boot using the dvd. But as the dvd is readonly, you won't able to save your changes. like downloaded software will get deleted once you reboot.
I suggest using a usb with persistance mode so that you can save your changes.
Many program can confugure your usb for that. One of them is Linux live Usb creater (LILI)
also remember using a os from USB will seriously reduce life span of the usb
At last i will say, you can install ubuntu without deleting windows 7. There are in many tutorial on web. If you need help on that just create a new question.