Spot on photos from a canon 350d
If you see bright circles or spots on photos taken with flash, and the lens appears to be clean, then this is probably caused by particles (dust, pollen, etc) in the air.
There's also an interesting thread on the CANON DIGITAL PHOTOGRAPHY FORUM at <http://photography-on-the.net/forum/viewtopic.php?TopicID=2912>
And an explanation of sorts at <http://www.fujifilmsupport.com/faq/tech/spot/spot.htm>
I really don't know if a slave flash would help, because I assume the A40 flash would still be operating, but if you could borrow one it might be worth a try.
Might be a wild goose chase, but you could try changing some small thing, just in case ...
... like trying a different flash mode - slow synch for example?
... or different picture modes - perhaps snapshot or infinity?
... or changing from AUTO to Program, or from Program to Manual, etc?
Otherwise it's probably a case of waiting for most of the dust to settle, taking more than one photo, or just trying to do without flash whenever possible in that sort of situation ...
Another suggestion I just came across is to use a Negative Ion Generator - used to purify air by precipitating out the particles. Don't know how well it would work - probably not very well if folk keep moving around - and sounds expensive, so maybe not worth buying one - but if dust is a regular problem, and you just happen to know someone that has one ...
poached from http://www.valsphotography.co.uk/information/Troubleshooting.html
hope this helps