I have had this issue in the past and found a couple of solutions.
First, make sure that the USB cable that you are using is the one that came with your unit. Standard 'USB' cables do not all work with the 8130 (it's all I own so not sure about other BBs)
Second, you may have to do a combo of all of the following:
1 - This is the one that almost always works - remove the battery from your BB and put back in and then see if PC recognizes it.
2 - Make sure that you have Desktop Manager closed before you connect it
3 - Reconnect device first to see if PC sees it - if you have a card in you should get the Windows pop up asking what you would like to do with it. You can just close that but at least you know that your PC is now seeing the device.
4 - Re-open Desktop manager and see if it is recognized.
If the above still does not work then re-boot your windows machine and try steps 1 to 4 above again.
This has helped me a lot. Good luck and let me know.