One of the biggest disappointments with the Canon Powershot S1 iS is its inability to focus correctly in low-light situations. That's because the digital camera lacks a focus assist lamp, as found on Canon's G-series of cameras. (The top-line Sonys go one better, by being able to take photographs in the dark with the assistance of its focus-assist lamp.)
There are solutions to the problem:
* Don't be zoomed out to 10x. At this zoom level, there is less "image" for the lens to focus on; the autofocus system needs contrasty edges to do its work.
* Use a strong flashlight to light up the subject.
* At Steve's Digicams forum, dafiryde suggets to:
1. Set camera to apature priority (Av).
2. Set aperture to 5.6.
3. Manual focus (MF) to approximately 6 feet.
4. ISO (aka ASA) to 200 (FUNC).
5. Flash to on or off.
6. Save this setting to custom mode.
Dafiryde is going back the the "old" way of taking photographs, back to when lens barrels were engraved with the depth-of-field range for each aperture. The larger the aperture number, the shorter the depth-of-field. By setting the aperture to a smaller number, more of the picture is in focus; by setting the manual focus of about 6 feet, then everthing in the picture is in focus from 2 feet to infinity.
(You could set the aperture to 8.0 and get everything in focus from 1.3 feet to infinity, but we're talking about low-light scenes, where the aperture needs to be larger to gather more light. Hence the ISO 200 setting to help boost shutter speed.)
To determine the depth-of-field for the Canon S1 iS at any size of aperture, check out the DOFMaster - DEPTH OF FIELD CALCULATORS.
For an explanation of depth-of-field, see this article on Hyperfocal Distance.