Do I have to install Acronis under Windows in order to use bootable CD?
I only intend to use Acronis to do OS image backup using bootable CD. I do not want to have it installed under Windows.
If I acquire retail (boxed) copy - will I be able to download bootable CD images from Acronis website just by using serial number from the box?
Sure, you can do that. The bootable Recovery Media, either on CD-R or USB flash drive, is entirely independent of the Windows package.
Yes, register your serial number at Acronis.com and you'll be able to download the latest Recovery Media .iso and updates when they appear.