I input your information into a pregnancy calculator (assuming that Oct 31 was the last day of the period, not the first day of it) and got a conception date of November 14, 2008. Keep in mind that this is an estimate and could be off by as much as two weeks.
If you remember how far along you were estimated to be any time during your first trimester, this will give you a better idea of your conception date. Ultrasounds done in the first trimester are much more accurate than ones done later in the pregnancy.