If it's been that long since a clean installation then I would go that
route. Otherwise from the command line;

echo %systemroot%

will return the one you're using.

To do a clean install, boot the Windows XP install CD-Rom. Setup inspects
your computer's hardware configuration and then begins to install the setup
and driver files. When the Windows XP Professional screen appears, press
ENTER to set up Windows XP Professional.

(Note: If your drive controller is not natively supported then you'll want
to boot the Windows XP CD-Rom. Then *F6* very early and very important (at
setup is inspecting your system) in the setup to prevent drive controller
detection, and select S to specify additional drivers. Then later you'll be
prompted to insert the manufacturer supplied Windows XP driver for your
drive controller in drive "A")

Read the license agreement, and then press the F8 key to accept the terms of
the license agreement and continue the installation.

When the Windows XP Professional Setup screen appears, all the existing
partitions and the unpartitioned spaces are listed for each physical hard
disk. Use the ARROW keys to select the partitions Press D to delete an
existing partition, If you press D to delete an existing partition, you must
then press L (or press ENTER, and then press L if it is the System
partition) to confirm that you want to delete the partition. Repeat this
step for each of the existing partitions When all the partitions are deleted
press F3 to exit setup, (to avoid unexpected drive letter assignments with
your new install) then restart the pc then when you get to this point in
setup again select the unpartitioned space, and then press C to create a new
partition and specify the size (if required). Windows will by default use
all available space.

Be sure to apply SP2 or at least these two below to your new install before
connecting to any network. Internet included. (sasser, msblast)
http://www.microsoft.com/downloads/details.aspx?FamilyID=049c9dbe-3b8e-4f30-8245
-9e368d3cdb5a&displaylang=en

http://www.microsoft.com/technet/security/bulletin/MS03-043.mspx
http://www.microsoft.com/technet/security/bulletin/MS03-049.mspx