In addition to security and pro user features, another big difference between XP Home and XP Pro is the multi-kernal.
XP Pro is able to handle two physical processors, while home is limited to a single physical processor (two logical processors, i.e. Hyper-thread).
Here is a link to Microsoft's answer to this question for hyper-thread techology.
Here is a quick table from the document.
Windows - Max Physical CPU Limit - Max Logical CPU Limit
XP Home Edition - 1 - 2
XP Professional - 2 - 4
Server 2003, Standard - 4 - 8
Server 2003, Enterprise - 8 - 16
Server 2003, Datacenter - 32 - 32