The Basic Stamp Homework board connects to the PC through a serial port. Modern PCs, particularly laptops, may not have a serial port but will have plenty of USB ports. To solve this problem, buy yourself a USB to Serial Converter.
There are many models and most but not all will run with the Stamp. Price does not matter.
One inexpensive model is the SABRENT 1 ft. USB to Serial db9 Male RS-232 (9-pin) Converter Model SBT-USC1M. This can be purchased from buyextras.com or from NewEgg.com for $9 plus shipping. It was tested by us and seems to work on most desktop and laptop computers including those running Windows XP and Windows Vista.
Instructions and a troubleshoorting guide for installing the Sabrent adapter cable.
Drivers for Windows 2000, Windows XP and Windows Vista (zip file, 1.7 Mb)
Or you can purchase the $15 USB adapter cable from Parallax that is guaranteed to work with the Stamp is recommended. Note that this device uses a USB A to mini B cable which can be purchased from Parallax or other sources.
If you have a usb-2-serial converter and still cannot connect to the stamp, try these usb troubleshooting notes from Parallax