AccessXP, as you've found, setting the value for a check box when the form opens works, but I question the reason to do so.
If the checkbox is FALSE, setting it to TRUE would not necessarily be correct, eh?
If you are opening the form with a NEW (to be added) record, the best way of doing what you seem to want is to set the DEFAULT VALUE of this field to TRUE, so any NEW records have that value ( -1 ) set in them.
It's usually a bad thing to alter field values 'just for the hell of it', e.g. not as the result of some user action. Opening a form and changing the value of something is generally not a good thing.
Correct me if my presumptions are false...(e.g., "0")..
"For a successful technology, reality must take precedence over public relations, for Nature cannot be fooled." - Richard P. Feynman