USB to RS-232 converters are a royal pain....
http://www.diyautotune.com/catalog/u...tune-p-67.html
is one I recently bought, it seems to work. The important thing is it includes a working driver, and you can download the driver from the web page.
The $2 converters on ebay, or pretty much any converter you find laying around, is not likely to work. Finding the driver is usually the biggest problem.
Although if you have old vex stuff, you could try the orange one that's included with the programming cable.