The default resolution for bitmaps is 120 DPI. You can change the default resolution by opening the printers folder, right clicking on the Win2PDF icon, and opening "Printing Preferences" (Windows 2000,XP) or "Document Defaults" (Windows NT). The resolution setting us under the "Advanced" section of this dialog.
Increasing the resolution will improve the quality of bitmap images, but it will also increase the PDF file size.