I use Sophos, but Ive had good results with Norton. There are lots of AV solutions around, theres bound to be one that suits your needs.

I often had this problem with an imporoperly configured Antivirus while sending emails with attachments. With all the scanning and problems at the computer it takes so long to get the data to the smtp server that the connection just closes. There may be an option to change the period of time before outlook decides that the connection has timed out. You might try looking for something like that and increasing the time (It may not even be there, Im just guessing).