USB to RS232 TTL
In my opinion the best USB to serial converter is FT232R. Windows Drivers are available there: http://www.ftdichip.com/Drivers/VCP.htm I've bought the following device for 10 bucks on popular Internet auction. Some nice features of that converter are: TTL 3.3V and 5 V RTS and CTS on golpins other signal lines (DTR, DSR, RI plus additional I/O lines) on standard raster holes on top and bottom of the converter board, you can solder goldpins for breadboard. The most simple test can be done by shortening TXD with RXD with simple jumper and using Putty to see if USB converter will do echo of what we are typing.