When you buy a new PC with pre-installed Win XP Pro, this is the OEM license. This license will 'die' when the PC dies (cannot be transferred to a replacement PC). OEM licenses only come pre-installed and should not be sold without a PC.
Win XP Pro (Retail) is for PC's that do not come pre-installed with Win XP Pro. This license will not 'die' when the PC dies and can be transferred to a replacement PC (but cannot be installed simultaneously to more than one PC).
There are PC's that come with pre-installed Win XP Home OEM. If you want to upgrade this to Win XP Pro, you should either get Win XP Pro Upgrade (Retail) or Win XP Pro Full (Retail). (If you get XP Pro Full retail, you cannot transfer the XP Home OEM to another computer).
Certain networking functions are not supported in Win XP Home (OEM or Retail).