Whenever Crystal Reports does not have permission to print to a given printer or the printer is viewed as invalid, the print button becomes disabled. If no printer is being selected before a report is viewed on the screen, then the default printer driver is used to generate the report.
Try changing the default printer and viewing the report again. If it works, then there is a problem with your driver or printer permissions.
If you are using Windows Vista or 7, what I've had to do sometimes was give the current user Manage Printers and Manage Documents permissions by going to the printer's property page and changing them there. This requires a reboot to fully update even though Windows never tells you so. If that does not work, try running the VB6 program in compatibility mode for XP.
I've also had this happen a handful of times when the user was trying to print to an Active Directory network printer and did not have Manage Printers permissions. Rarely I've seen this happen on a corrupt print driver or a driver intended for a different version of Windows. If none of the above works, I would try downloading the latest version of your print driver and reinstalling the printer.