I assume you just copy wallet.dat and put it somewhere else, and then you can restore it when you want to spend the bitcoins in it by just copying the file back to the directory it normally resides in, but won't the client stop working if you take away the wallet.dat file? If it just creates a new one, what happens to transactions in the new wallet.dat when/if you over-write it with a backup? Please explain, I'd rather not plunge confidently ahead under the assumption that I know precisely how to back up because it's all self evident and obvious- as I was about to do, only to find I've buggered it all up.