Your camera may have become one of the other drive letters.
If you open up "My Computer" you normally see a "C" drive which is your hard disk and maybe a "D" drive for your CD.
You may have other letters depending how many hard drives or CD drives you have.
Your camera will usually then become the next "free" letter, such as E, F, G or whatever.
I have both Windows XP and Vista and when I plug my camera in I get a window up which asks what I want to do now - and one of the options is to download the pictures.