I'd like to install Ubuntu 11.04 on an existing Windows Vista installation. However, if I see that Ubuntu doesn't fit my needs, I'd like to have the option to remove Ubuntu and reinstall Windows Vista on top of it. Is that possible with a "Windows Anytime Upgrade" disc that I got from my manufacturer (Toshiba) and with a genuine license key for Windows?
Yes, Windows will have no problem overwriting your current configuration. It doesn't even try to play nice if you have a dual-boot setup either.
Why not to use both Operating Systems? Ubuntu can be installed without removing vista, and if you want to go back to have only vista, you just delete the partition where you have ubuntu; that way, you save the time installing vista.
I'd suggest installing Ubuntu as a dual-boot setup, where you have both operating systems available. The Ubuntu installer can resize the existing windows partition to make room for Ubuntu to fit on the same harddrive. Ubuntu can mount the windows drive also, so you can store files on both partitions.
About reinstalling windows, I'm not 100% sure if it can be done with an upgrade disc. Typically, upgrade versions require some version of windows to be present at the computer before they allow themselves to be installed.