On Linux, the answer would be yes: you can copy an installation from a system to another one (assuming both use the same architecture (as in 32 bit vs 64 bit)). If the target system didn't have a TeX Live installation before, then you would have to add the path of the installation to your system's (or user's
PATH) as with every new TL installation.
On Windows, the answer is slightly more complicated, since your TeX Live installation consists not only of the files in the tree, but also various registry entries for file associations, menu entries, possibly desktop shortcuts too.
Theses things almost never change between major (yearly) versions of TeX Live, so most of the time just copying the files in
C:\texlive will be enough for update. For the remaining 0.1% cases, you have two options. The pragmatic one: wait until you notice a problem like a missing file association, then figure out which package it belongs to, and run
tlmgr postaction install fileassoc <name> to install the missing association, and similarly for desktop shortcuts if you want them. The paranoid one is to watch the list of packages updates and take the actions needed without waiting to notice a possible problem.
For installation on a system, things are a bit more complicated since there are also the
PATH and menu to handle, as well a registering TeX Live as an application in the 'install/remove application' section of the control panel. Here I think your safer bet is do to a regular installation using the installer. Note that if your existing installations are full TeX Lives, they can serve as a source for other installations: you should find a copy of install-tl.bat at the root of the installation, so the installer doesn't need to download the packages again from a mirror.
Note that another option for faster installation and upgrade on multiple machines is to maintain a mirror of tlnet, make it accessible on your local network, and use it as the source. For this, rsync is the preferred option (it is available on windows too), see http://tug.org/texlive/acquire-mirror.html. By the way, rsync is probably the most efficient way to synchronise your TeX Live trees across your machines if you choose to do so.