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 about $10 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.
Driver for all flavors of Windows (zip file, 2.3 Mb). Unzip and run the installation file.
If the adapter is still not recognized when you plug it in, try this method. Download the Sabrent driver CD files (zip file, 20 Mb). Unzip. Run Setup.exe. Click "USB TO RS-232 CABLE DRIVERS". Connect the adapter when asked to plugin the PL2303 COM Port.
Confirm that the com port is working by opening the Windows Device Manager
(right-click on My Computer > Properties > Hardware > Device Manager. Open
the Ports (COM & LPT) entry. There should be an entry with a COM number
indicating the USB to serial adapter cable.
Another cable option is the 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