This is an issue in Screenwriter 2000 (4.x) and is usually due to a faulty or corrupted printer driver. Screenwriter uses the printer driver to get correct paper size information to calculate margins accurately.
There are two main solutions to this issue:
Solution #1 - (if multiple printer drivers are installed in the system)
Under Start > Control Panel > Printers & Faxes, right click a different printer (NOT a fax or image driver) and set that as the default printer for the computer.
Solution #2 - (if only one printer driver is installed)
Delete the printer that is listed and reinstall the software for it. A good idea would be to go to the printer manufacturer's website to download and install the latest driver for the printer, rather than using the CD that came with the printer.
This issue has been fixed in Screenwriter 6.