Don:
I have used a great program called WinHexCom. It works especially nice becasue it is in support of stuff going on outside the PC in which it runs. (The progam that Marshall recommemded is a bit of a different animal).

WinHexCom will actually let you connect to two COMM ports in the PC and monitor the RxD traffic from both ports. This is useful for monitoring a network that uses RS232 or RS422. In your case with RS485 you should be able to get by with just one COMM port and an adapter from RS485 to RS232 from someplace like B&B Electronics.

Here is the link to the WinHexCom information and
download page...

http://aerospacesoftware.com/shareware.html

...This program is shareware so you can download it for free and then pay a nominal fee should you elect to get a license for it. (Interestingly they take PayPal for payments via their web site).

Michael Karas