In Vista it's right click Computer and choose manage. On the left is Services and Applications. Choose Services and find WIA. It should state started and automatic. If not running right click it and choose start. Can't remember how to get to it in XP. It's been a while now.
It's a long, long time since I used Canon software, but if I remember correctly once deleted if not done with the Canon installer/uninstaller it can leave traces that stop a new installation. I'm almost sure at that time I had to reformat my drive, then installation went ahead without problem.
Adobe had the same problem with either CS2 or CS3 which again necessitated a reformat of the drive.