Well, it's difficult to make sure we have the exact same version you do, and indeed my version is 2.3.2 instead of 2.5.5. I'd say your best bet is to install the version you can get through Debian's tools.
Boot into knoppix or another live CD and mount your hard drive there, then find the libc package you downloaded in /var/cache/apt/archives (it'll probably start with libc6 and it should end with .deb). You can copy that deb file somewhere else (I'd say /root is good), make a folder (under /root) "mkdir /root/libcfolder" to extract it to, and extract it there : "dpkg -x /root/libc6....deb /root/libcfolder". Then you can get the exact file that was missing (it'll be inside /root/libcfolder), and copy it wherever you need it to be. If you've cleared your cache of .deb files recently, then you'll have to find your version of the package and go download the deb file again at http://packages.debian.org/,
and then do the process above.
By the way, As a general rule of debian / most linux systems
, I would never manually touch the files in /usr/(anywhere) except /usr/local, unless you really know what you're doing. That's the domain of the package manager, and you should generally only try to modify those folders by sending commands to the package manager.
If you are an OO programmer, think of the folders under /usr (again, except /usr/local) as private variables of a class. Don't modify them directly, call the interface functions instead (apt-get and dpkg). If you're not a programmer, then I don't know any good way to describe it, and you can ignore this paragraph.
Anyways, good luck getting your system back!