This one has me totally baffled; The checkout on my website will fail sometimes depending on what is installed on the computer that is used. This is definitely not a browser problem. On my computer which I use for web developing, it works perfectly every time; on my dad's new Dell, it fails every time with an ominous error message (included below).  Why do you think that is?

V

PS: the oscommerce checkout mentioned above is only 1 page long and utilizes ajax and javascript.

---message reads---

Credit Card Error!
   The first four digits of the number entered are: <br>If that number is correct, we do not accept that type of credit card.<br>If it is wrong, please try again.

Thanks!!! I'll try out your suggestions - starting with the javascript - and let you know what happens.

I've never heard of cURL caring what browser or what OS was on the visitor's PC. As far as I know, it's strictly on the server. If it works for some, it should work for all, unless there's code somewhere that asks what browser software is being used and tries to do different things based on that. I don't recall seeing any such code in osCommerce itself, but it's possible that Authorize.net might do it. I would look for some difference in browser settings between the XP Pro systems that work, and other systems that don't work -- particularly who allows JavaScript and what browsers are being used. Do you have a php.ini setting to explicitly turn on  register globals? If you don't, the default on most servers is off. Don't forget to run a page with phpinfo(); to list all the settings in effect -- it will tell you if cURL is enabled and whether register globals are on or off. I've never worked with Authorize.net, so that's all I can think of at the moment.