Can't Download From POP Account When Logged Into Domain

Any suggestions would be appreciated...

I am unable to download e-mail from my POP account when I am logged
into my domain (I can send e-mail fine). When I am off of the domain
(i.e. at home) downloading works fine. My stats are...

Dell Latitude D620
Clean Install of Vista with all updates current
Office 2003 (Outlook 2003) with all updates current
The POP account is the only e-mail account set up (we are not using
exchange)

SBS 2003 is my Domain controller with all updates current. I did have
to download a patch from MS to get the Vista machine registered on my
domain.
This is the only machine running Vista - the other 4 on my domain are
XP and obviously working fine.

I currently do not have any anti-virus software installed on the Vista
machine but I am running MS Defender (which came preinstalled from
Dell)

I tried turning MS Defender and the Firewall off but that did not have
any effect.

Repeated reboots had no effect

I am guessing that there has be something (a policy perhaps) that is
interacting between my domain controller and Vista machine but I did
not see anything in the policies relating to this.

One other fact that may or may not be related, the first message in
the queue had an Excel workbook attachment (about 214Kb in size)

When I set up Outlook to download from another POP account (Yahoo), it
downloaded the messages from that account first then they both hung up
once it got to the messages on my original account. The messages from
Yahoo never got marked as downloaded. Because when I hit send and
receive again, the Yahoo messages downloaded again. This happened 3
times (same messages came down three times) before I took the Yahoo
POP account of the send/receive group.