I don't think most usb to serial adaptors work well. The only one that seems to work reliably is the Keyspan model USA-19HS.

A cheaper solution might be a header cable. While most new computers dont' have serial connectors, they usually have the serial port header on the motherboard. You just need to get the adaptor. It is usually an slot cover with a serial port on it and a ribber cable that plugs into the header. Here is an example:
[Link: cgi.ebay.com]