Counting multiple conditions

Hey all,

I could use some help with this one.
My worksheet contains a database of systems that have been installed since 6/08. Each system has its own unique serial number, however, this serial number appears more than once.
I'm trying to count the number of systems that have been installed since 9/08/08 but I do not need Excel to count each s/n entry, I only want it to return a count per s/n once to get a total of all installations since this date.
I have attached the sheet in pk zip format.
Any thoughts?
Thanks in advance.