Memory Access Error in HttpOpenRequest

On my developement machine running WinXP, I have no problems. But on a test machine running Win2k, my program crashes, and the log that is produced by Windows indicates that there is a memory access error. I have traced down the fault to the following offending line of code, a winInet API call.

.......
If HttpSendRequest(hURL, "", 0, postBuffer, Len(postBuffer)) Then
.......

postBuffer is a URL encoded string with valid data in it, I have checked that. hURL is what is returned from HttpOpenRequest (checked that it is not zero). I even tried making it Len(postBuffer)-1, just to see if maybe the indecies were off, but I still get the error on Win2K. I have not tested on even older systems, or for that matter, any other Win2k machine, but it seems fine on the WinXP boxes I have tested it on.

Anyone have any ideas on what to try next?