How to use serial port device if PC has not serial port?

Dear Team,

i want to know abot serial port comminication..in my IBM THINKPAD R51,has not serial port but at some i must need it for loginn in Routers and others some equipment,

so please guide me how can i use serial port from this laptop.?

laptop has only DB25 and db15 pin connector.

pls guide me..and give specific answer..

Naransinh Chauhan.