Digital cameras really are battery hogs (especially cameras that use only 2 AA's). Some brands of batteries will have problems just starting up the camera. This may be true even if they're alkaline, brand new, and just out of the package.
Recommend rechargeable NiMH batteries only for your camera. Believe Canon even states this in their user manual. Walmart sells these for around $7 for a package of four (about $15-19 for the batteries with charger). Keep in mind they'll save you big bucks in the long run over alkalines, AND they'll last for at least 100 pictures per charge (and probably many many more). You'll be very pleased with their performance, and may slap yourself for not buying them sooner. When at the store, look on the package for a power rating of at least 2500 mah.
If the above doesn't work, here's some other things to try: