Sometimes the PC assigns a drive letter to removable devices that you may already have mapped to a network shared drive or other device. If that's the case, the other device will always take precedence over your BlackBerry, because drive letters are first-come, first-served. It drove me CRAZY for weeks, because I knew that USB was detecting the device, but it would NOT show up in Explorer. My cubie figured it out.
Right-click on My Computer and select
Manage. Go to Disk Management. If your BlackBerry shows up here as a removable media device, right-click on it and select "Change Drive Letter and Paths." Select a drive letter that is not already mapped to a network drive or other device on your PC. Refresh Windows Explorer, et voila! Thar she blows! Hopefully.