I didn't write this, found it on some forum, just make sure you copy all the files on your office xp corp version disk to your hard drive. good luck!
SP1 Update:
firstly it will be easier to just uninstall your current office xp
installation.
Ok run this in the command box (start/run)
D:\setup /a
this will copy the stuff from the office cd to the hard disk
this creates a administration installation point on your HD (c:\office)
then download the service pack from this place (40megs)
http://download.microsoft.com/download/officexpstandard/sp/oxpsp1/w98nt42kme
xp/en-us/oxpsp1a.exe
open the exe and extract the to files to c:\config.msi
create a "temp" dir in C drive
then run these 2 commands in the run box again (separately 1 after the
other)
msiexec /p C:\Config.Msi\MAINSP1_Admin.msp /a C:\Office\PROPLUS.MSI
SHORTFILENAMES=TRUE /qb /L* c:\temp\1.log
msiexec /p C:\Config.Msi\OWC10SP1_Admin.msp /a C:\Office\OWC10.MSI
SHORTFILENAMES=TRUE /qb /L* c:\temp\2.log
it will seem like a quick operation but open the log files created to see if
the msi patch updates were successful (right at the end)
then reinstall office xp from the c:\office location like normal
and now you have a office XP installation which is fully updated
update: this works for enterprise/corporate version (possibly only)