You would need to purchase an individual Windows XP
license for each installation on a different computer.
So, you would need to purchase two (2) upgrade versions
of Windows XP.

Does anyone know if Windows XP Home Upgrade can be installed on 2 different
computers? We have 2 computers that we would like to install it on, but we
don't want to spend $100 if we can't use it on both pc's.
Thanks!

In strict accordance with the End-User License Agreement that you would
agree to as you installed Windows XP, nope.

"1. GRANT OF LICENSE. Microsoft grants you the following rights
   provided that you comply with all terms and conditions of
   this EULA:

   * Installation and use.  You may install, use, access,
     display and run one copy of the Product on a single
     computer, such as a workstation, terminal or other device
     ("Workstation Computer").  The Product may not be used
     by more than two (2) processors at  any one time on any
     single Workstation Computer."

and...

"    * Storage/Network Use. You may also store or install a copy
     of the Product on a storage device, such as a network
     server, used only to install or run the Product on your
     other Workstation Computers over an internal network;
     however, you must acquire and dedicate an additional
     license for each separate Workstation Computer on or
     from which the Product is installed, used, accessed,
     displayed or run. A license for the Product may not be
     shared or used concurrently on different Workstation
     Computers."

No Microsoft Windows Operating System (95, 98, ME, NT, 2000, XP, 2003,
2003R2, Vista... etc and so on..) has ever granted you the right to
install/utilize the operating system on more than one computer without
having more than one license.  Windows XP just actually put in some form of
'accounting' (called 'activation') that makes it more difficult for you to
utilize a single license on more than one computer - so that you not only
agree to the typed out agreement by clicking on 'I Agree', but you also have
to 'agree' again when you activate Windows XP.

What does that mean to you?

In order to install Windows XP and legitimately agree with the terms of its
use - you must have purchased two licenses/copies of the OS.