Here are a few tips that may help if your PC is unable to detect your BlackBerry smartphone.
1. Install the desktop manager software – when installing the Desktop Manage software that came with your device be sure to Not have the BlackBerry connected to the computer during this process.
2. Always use the USB ports from the back of the computer; never use a USB hub or a docking station USB port.
3. If you have another UBS cable you can try doing that to make sure it is not the cable causing the issue.
4. If you are using a laptop computer disable the power management settings.
a. For Windows-XP and Vista do the following to disable power management on the USB ports
i. On the computer, right-click My Computer and select Properties.
ii. Click the Hardware tab, and then click Device Manager.
iii. Expand Universal Serial Bus controllers.
iv. Right-click the USB Root Hub and select Properties.
v. Click the Power Management tab, and clear the Allow the computer to turn off this device to save power check box.
vi. Click OK.
vii. Repeat steps i. through vi. for each USB Root Hub listed in BlackBerry Device Manager.
5. If you have uninstalled and reinstalled the Desktop Manager Software trying using the following knowledge base article to do a “clean uninstall” of the software,
KB02206-How to perform a clean uninstall of the BlackBerry Desktop Software
6. The existing USB driver installed is for an earlier model of the BlackBerry smartphone.
a. Manually upgrade the USB driver for the BlackBerry smartphone by completing the following steps:
i. Right-click My Computer and select Properties > Hardware > Device Manager.
ii. Under Universal Serial Bus Controllers, verify that the BlackBerry smartphone is found.
iii. If an exclamation mark is displayed, right-click the BlackBerry smartphone and select Update Driver.
iv. If the Hardware Update Wizard prompts you to use Windows Update, select No not this time and click Next.
v. Choose Install from a list or specific location and click Next.
vi. Choose Don't search. I will choose the driver to install and click Next.
vii. Click Have Disk and then choose Browse.
viii. Browse to C:\Program Files\Common Files\Research In Motion\USB Drivers.
ix. Double-click on RimUsbNT.inf, click OK, and then click Next.
7. If you have another PC try using that PC to see if you able to get the device connected
8. If you do not have another PC then try creating a new user account on the PC and then install the Desktop Manager software on that new profile.