You need InfoPath to fill out forms.

from
http://www.microsoft.com/technet/prodtechnol/office/office2003/plan/inpthfaq.mspx:

Q. Do employees need to have InfoPath installed on their PCs in order to
fill out forms created with InfoPath?  Is InfoPath required for viewing or
reading an InfoPath form?

A. Information workers will need to have InfoPath 2003 installed on their
PCs to enter information into InfoPath forms and to take advantage of the
rich client functionality such as structural editing, business-logic
validation, rich-text formatting, AutoCorrect, and spell-checking.  That
said, you can read an InfoPath form that has been saved to .MHT in your
browser or within an HTML-enabled e-mail client, as the body of your mail
message.  Because InfoPath generates XML belonging to any customer-defined
schemas, other software could be used to reuse and display that XML
document.

Q. Do I need to touch every desktop to deploy InfoPath?  Can you host an
InfoPath form in the browser?

A. In order to fill out or design an InfoPath form, InfoPath needs to be
installed on the desktop.  However, once you have deployed InfoPath, you can
easily roll out the form templates that users need to use InfoPath.
InfoPath provides transparent deployment and maintenance of solutions via a
Web-like model that downloads solutions on demand.  InfoPath form templates
are downloaded silently and seamlessly to the client when a user clicks on a
link to an InfoPath form or clicks on an InfoPath attachment.

Other Links:
http://blogs.msdn.com/philoj/archive/2004/03/16/90636.aspx
http://blogs.msdn.com/infopath/archive/2004/03/25/96432.aspx

--josh bertsch