Can't seem to send wmv files

[Slam]

I've just upgraded PC (and used 'file & settings transfer'). Outlook runs
fine but when I try and send/forward a message with a .wmv attachment it just
stays in the outbox.

I'm running XP Pro/SP2, Outlook 2003 from Office 11.

(Interestingly, don't know if it's related but I tried to run an Ofice SP
upgrade today but it fails every time, even if I split up the downloads to
apply.)

Thanks.