When people using IE 7 or 8 under Windows XP try to submit my form, they receive a message saying something like "a script is slowing down your computer, do you want to stop it?". If they click no, the form is sent, but if you click yes stop it (which is the case of the majority), I won't receive the form. Is there a way to avoid having this message?
I would guess that the form has large number of fields with validation that may take some time to process on older systems. Not sure if it's feasible to try to reduce the form size and remove validations and required attributes to see if it helps.
Can you also send the form url to firstname.lastname@example.org so we can investigate more.