Using mac and Windows XP - How to upgrade from XP Home to XP Pro Edition

I have a client that bought a new iMac and Windows XP Home with SP2 to run on it and access her work computer. Now her work administrators tell her she needs the Professional edition of XP to access the Work network.

What is the cheapest and/or easiest way to upgrade XP Home to XP Pro? She already spent the $299.00 for XP Home. Does she now have to spend $99 (or whatever) to buy XP Pro Upgrade?

or can she get to Pro by downloading and installing updates from microsoft's website?

Thanks.