Why Outlook unexpected error w/attachment?

Whenever trying to send an attachment through outlook a box comes up that says the following:
An unexpected error occurred: not enough memory available to complete the operation.
Close other programs to make more memory available, and then try again.
The computer is running Windows XP and Office 2007. There is 3GB of RAM installed on the machine.
The file will send after the machine has been rebooted.
