Got a bit of a weird one here.
On the order form, if you click "select date", what does it do for you? For me in IE 6 and 7, I get an error (using IETester)
Does it work ok for other people on the stand-alone versions? (I'm wondering if its a problem with the popup stuff not being liked with IETester)
FireFox says showCal is undefined but it is present in the linked script and Opera runs it. Judging from the level of validation errors, it's probably a parsing error.
I would fix the errors first: validator.w3.org