How can I set up a computer with access to 2 different networks so
that for file sharing and email it uses one connection, and for
internet and anything else external it uses another connection?

Ive got a home network set up using Belkin wireless gateway router
connected to cable broadband modem, with a desktop pc running WinXP
Pro ethernet connected to router, and Laptop running WinXP Home
wireless connected to router. Ive also got my partners PC running
WinXP Pro wireless connected to router, but this PC has ADSL broadband
connection of its own. This connection is via a 'dial-up' USB modem.
When the 'dial-up' modem is not connected, this PC can send and
receive e-mail via the home network fine, but when the 'dial-up' is
connected, Outlook only tries through this connection. How can I
force it to connect only through the wireless connection? Internet
explorer is set up to connect 'always using dial up', but outlook is
set up to connect through LAN.

Any help will be very much appreciated.

Bournemouth UK