Since I don't have exactly the same Sierra Wireless card as you, my setup will be a little different. Look at a screenshot from my setup and see what I have. Note that my Sierra Wireless 860 AirCard is listed under Modems, Multifunction adapters, and Network adapters in Device Manager.

http://img85.imageshack.us/img85/5318/networkconnectionsmi3.jpg
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Take a look at the following and see if this is of any help. This applies primarily to WiFi connections, but still there is useful information there.

How to troubleshoot wireless network connections in Windows XP Service Pack 2

http://support.microsoft.com/kb/870702/en-us