Re-installed PC can not see Exchange Server

I have have a SBS 2003, and two different PCs, 1 running XP Prof, the other
XP Home. I have just had to re-installed XP on both (for differnt
reasons)...however, the same problem is occuring on both:

Going via Control Panel - Mail Setup - Outlook - E-mail Accounts (o) Add a
new e-mail account - MS Exchange Server -
MS Eachenge Server = host.HOME.local
[x] Use Cached Exchange Mode
Username = lachlan
then I select the [Check Name] button, and I get the following error
message: "The action could not be completed. The connection to the MS
Exchange Server is unavailable. Outlook must be online or connected to
complete this action."

If I uncheck the [ ] Use Cached Exchange Mode, I get a slightly different
message: "Outlook could not log on. Check to make sure you are connected to
the network and are using a proper server and mailbox name. The connection
to the MS Exchange Server is unavailable. Outlook must be online or
connected to complete this action."

With my remaining PC (runing XP Home), I am able to run Outlook and connect
to the Exchange Server using the SBS 2003 Username 'lachlan'. I have checked
(and triple checked the Workgroup, as well as the MS Exchange Server name and
other settings), and each PC can see the SBS 2003 in terms of File Server and
Network Places, as well as print to the printers connected to the SBS 2003.

So, what have I missed out on setting up so that these PCs can see the
Outlook server?