I looked in the XP Help and fooud the following. It states for NT but the XP has the same settings.
Creating a Custom Form
To create a custom form for printing to a non-standard paper size in follow these steps:
NOTE: You cannot create a custom form unless you have first installed a printer driver that supports non-standard paper sizes. For example, many printer drivers for dot matrix printers support custom paper sizes.
Click Start, point to Settings, and then click Printers.
On the File menu, click Server Properties.
In the Print Server Properties dialog box, click the Forms tab.
Click to select the Create a New Form check box.
Type the name of the custom form in the Form Description box. Under Measurements, specify the form's dimensions and margins.
Click Save Form, and then click OK.