I got the following while trying to install Doom 3 (demo version) to a computer. It meets the requirements to play the game, but it isn't anything spectacular (it basically just meets them). It's a Dimension 4300.

Anyway, the message says this:
Could not initialize installation. File size expected=483603646, size returned=483606926.

The numbers returned are always the same, and a reboot didn't help either. It comes up right as the wizard starts. Any idea what this means?