I assume that my Ubuntu disk, when I finally pluck up the courage to boot it up
The Standard Ubuntu / Kubuntu / Xubuntu install disk* allows you to boot into a "virtual" installation, running off the CD, that is not actually written to disk, so that you don't disturb your existing installation - whatever it is - giving you the opportunity to "try" the OS before installing it fully. You can actually access the "Windows" hard disks - even NTFS - which is how a number of useful Linux-based recovery (and Windows password changing) applications work!
You could then do a dual-boot install, assuming that you wanted to go ahead with the installation, which would still leave you the option to go back to Windows. Although NTFS write-access is still a bit iffy under Linux (MS have never released the proprietary NTFS spec. (this may change / have changed recently, not sure).
I bought a 2nd internal hard disk and mounting caddy for my laptop, so that I can swap completely (undo one screw, slide out disk, replace, do up one screw - 1 minute job). This suits me because I just use Kubuntu for learning purposes at this time - I am still firmly anchored to XP for many reasons.
SD
* - Except the Alternative Install image, which is designed for systems with insufficient RAM for the "trial" install.