I have had the same problem, in the last several days, sending attached files. I run on a dsl connection, and could not send two files, half a megabyte in size. So far, I have followed the advice and cancelled the scan of outgoing attachments - and I will wait to see if the uninstall / reinstall works. I just thought it would be of some value to report the same problem on another system.
I run Windows XP, using a dsl connection (1500 / 128), and I have been sending attachments before the last week or two with no problem. I can't really pinpoint when this problem developed.