Windows XP/Outlook 2000/Norton Internet Security2006

I have had an issue with POP3 resolution since getting hit with a
combination of Backdoor.Trojan and SpySheriff, which NIS supposedly fixed.
Upon initial boot Outlook can resolve the POP3 servers and send/recieve mail
successfully. If the user is logged off and back on, Outlook indicates that
the POP3 server name can not be found. If I open a DOS prompt I can ping the
POP3 servers by name. Still, Oulook indicates the same message. The PC has
to be rebooted in order to connect, but it will only do it once successfully.

Any ideas of what could be interfering with the POP3 name resolution?