Importing is never the way to transfer Outlook data between instances of
Outlook.  One reason is what you see.  You'll have to open each contact,
modify it (by changing a letter in the name to itself, perhaps), and save it
again in order to make Outlook add the birthdays to the calendar.

Imported contacts need to be "saved" for the b'days to take effect.
Create a temp Category, add those contacts to the new category... remove
them from the category.

I am using Outlook 2003 and XP
Recently I imported my contacts from another computer.  Some of those
contacts contain birthdays. However after importing these birthdays do not
appear in the calendar.   Any suggestions would be appreciated

Thanks
Nick