This is wildly simple but a really big help for me, I upgraded my computer, the last one on Xp to windows seven a while ago and found out that my Canon 20D and various other older ones were unsupported for drivers on Vista and Seven, not just hard to find drivers, they weren't even supported to view files with the new canon drivers.
On top of that my card reader has went missing...
Step 1: Enabling the camera to communicate.
Go in to the menu settings of the camera:
Menu>communication>select PTP instead of normal
Step 2: Notes on using this mode.
Once connected the camera will be permanently busy, unlike normal mode, it is however accessible as a drive now instead of just being an error message in the corner.
So far I have found no way of using the capture and camera control functions with PTP mode, I don't know if it's possible but with this camera the EOS utility for exactly that doesn't work on Vista or Seven