I want to try ubutu on my Windows 7. I understand I can download ubutu on a CD or on USB and use it on my laptop with Windows 7 without installing ubutu on my laptop. Is it correct and is there any problem that may screw up my wins7 later? Thanks for advice

Yes, you can create either a live CD or USB without installing to the hard drive. It will not screw up windows.

http://www.ubuntu.com/download/deskt...re-you-install

Watch and learn.

https://www.youtube.com/watch?v=fU3zCL5g_7E

The Best and safe option is to download in win 7 Virtual box then From virtual box everything is so easy to do.
google instructions how to install linux in win with virtual box.

I have downloaded Ubuntu on a DVD but when reboot my WINS7 computer, it doesn't boot from the disk. Do you know any reasons and how to fix it. I even press F12 to boot from disk but still doesn't wok. Your help is appreciated.

Did you burn the image as a iso? Not data.
Did you md5sum check the Ubuntu iso before burning the cd.

If not understanding me.
Youtube has the answers to what I am saying.

"How to md5sum in Windows 7"
"Burn a Linux iso image in Windows 7"

That's an interesting way of putting it. It's a two step process. You download the iso image to your hard drive then you burn it as an image to the DVD. Whatever software you are using to burn to the DVD should have an option on its menu to 'burn as image'. You would then set the boot priority in the BIOS immediately after booting. You should see a message at the bottom of the screen telling you which key to tap to do so. That would depend on the manufacturer. Windows 7 is just the operating system software, its not a windows computer. Major manufacturers are Dell, HP, Compaq, and many more and the key you tap depends upon the hardware.