Cybmole, turn UAC back on. There's no reason to turn it off, and doing so can cause other weird problems. Also, the "calibre2" folder name is correct. I assume it's named that way because at some point calibre had to break backward compatibility and Kovad chose to rename the installation directory to avoid any odd upgrade scenarios.
Chirotor, is this a personal machine or a work machine? Is it part of a domain, or stand-alone? Just because you're local administrator doesn't necessarily mean you have control of the computer. If the machine is part of a domain, a domain administrator can set policies that disallow things like installing new applications even if you are the local admin. It's possible those policies got enabled accidentally in your local security policy (google for the error you're getting, you'll see how to fix it), but that shouldn't be the case normally.
If all else fails, get an installation log. Open a cmd.exe window as administrator, and then run "msiexec /i <the calibre msi> /l*v calibre.log", replacing "<the calibre msi>" with the name of the calibre msi file, and then share the contents in calibre.log (warning -- it'll be rather large, so it may or may not attach to the post and you might have to put it up on dropbox or similar and link to it instead).