Just boot from the Windows XP CD (change the BIOS boot order if
necessary to accomplish this) and follow the prompts for a clean
installation (delete the existing partition by pressing "D" when
prompted, then create a new one).

When it doesn't find a previous qualifying version installed, it will
prompt you to insert its CD as proof of ownership. Just do so, and
follow the prompts.

You can find detailed instructions here:
http://michaelstevenstech.com/cleanxpinstall.html

or here
http://xphelpandsupport.mvps.org/how_do_i_install_windows_xp.htm

or here http://windowsxp.mvps.org/XPClean.htm

or here http://www.webtree.ca/windowsxp/clean_install.htm

But if I were you, I would do an upgrade installation instead. My
recommendation is to at least try the upgrade, since it's much
easier than a clean installation. You can always change your mind and
reinstall cleanly if problems develop. Note that, unlike with previous
versions of Windows, an upgrade to XP replaces almost everything, and
usually works very well.

   It's quite possible to perform a clean installation of WinXP Pro
using the Upgrade CD, provided you have the true installation CD for the
earlier OS.

    Simply boot from the WinXP Upgrade CD.  You'll be offered the
opportunity to delete, create, and format partitions as part of the
installation process.  The Upgrade CD checks to see if a qualifying OS
is installed, and, if it finds none, it asks you to insert the
installation media (CD) of that OS.  Unfortunately, an OEM
"Recovery/Restore" CD will not work for this purpose; you must  have a
true installation CD, complete with the "\Win98" folder and  *.cab
files, or the "\i386" folder of WinNT/2K.

    Before starting, however, have you verified that all of your PC's
hardware components are capable of supporting WinXP?  This information
will be found at the PC's manufacturer's web site, and on Microsoft's
Windows Catalog: (http://www.microsoft.com/whdc/hcl/default.mspx)

    You should also take a few minutes to ensure that there are
WinXP-specific device drivers available for all of the machine's
components.  There may not be, if the PC was specifically designed for
Win98/Me.  Also bear in mind that PCs designed for, sold and run fine
with Win9x/Me very often do not meet WinXP's much more stringent
hardware quality requirements.  This is particularly true of many early
models in Compaq's consumer-class Presario product line or HP's
consumer-class Pavilion product line.  WinXP, like WinNT and Win2K
before it, is quite sensitive to borderline defective or substandard
hardware (particularly motherboards, RAM and hard drives) that will
still support Win9x.

Upgrading to Windows XP
http://aumha.org/win5/a/xpupgrad.htm