Vista Office? Do you mean Windows Vista, Office 2007 or both?
It helps if you don't quote unassociated model numbers! I assume the XPS400
is a Dell desktop PC? According to the spec. it includes a fax modem. That
being the case, I suspect Vista will provide a driver that will enable it to
be used from Windows (ask a Vista forum) . Thereafter sending fax is simply
a matter of connecting the modem to a telephone line and 'printing' to the
fax driver.
A quick check at Google reveals that Vista offers fax compatibility in its
business, enterprise and ultimate versions. For lesser versions there will
be third party applications - try Mighty Fax?

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Graham Mayor - Word MVP
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Word MVP web site http://word.mvps.org
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> I recently gave up my mutifuction HP Office Jet (which included a
> fax) for a Dell
[quoted text clipped - 4 lines]
> Will I have to pay for a "fax service" (it was free with the
> OfficeJet.