Deleted usr/lib/libxml2.2.dylib -- computer won't start Long version:

I mistakenly deleted usr/lib/libxml2.2.dylib and now the computer will no longer start (it just stays in startup mode indefinitely). Would the solution be to reinstall the operating system? The computer came with Snow Leopard and have since upgraded to Mountain Lion. If I reinstall the Snow Leopard version (given that's the one I have a disk for) am I in danger of losing anything on the hard drive?

Short version:

I actually have the correct version of libxml2.2.dylib saved on the Desktop, so if I could get into the computer I may be able to fix the problem.
If you still have the Snow Leopard Installation DVD you should be able to solve the issue by booting from DVD and then open Terminal to copy the missing file from your desktop (which will be in /Volumes/however-your-disk-is-called/Users/Dash/Desktop) back into the correct location (/Volumes/however-your-disk-is-called/usr/lib/).