I had this problem and "did the Google". I found that this was a recurring theme so wanted to mention my findings.

After installing BB MediaSync PC application, my BB 8530 no longer showed up as an available mass storage device. If I checked the properties of my drives, it did show up as a valid USB storage device but WindowsXP apaprently couldn't assign a drive letter to this.

Much of the net chatter was about drive letter conflicts, but I think that was only anectdotal.

What I found was that if I launch the MediaSync App, it may or may not fail (depending upon timing) to find the device at first, but then with MediaSync running, I open My Computer. At that point the drive conflict is resolved and MediaSync "finds" the device and WindowsXP assigns a drive letter to the device as a USB drive.

I'm not sure why this is, but I think it has something to do with MediaSync hijacking the USB connection and causing a conflict.

So, when I connect my device, my Desktop mgr syncs my data but won't show up in My Computer. At that point, I open MediaSync, close MyComputer, re-openMy Computer and leave MediaSync running in the background minimized. At this point I have a drive letter.

A second method I've seen work is to open MyComputer and I have sections entitled Hard Disk Drives, Devices with Removable Storage, Other, Scanners and Cameras.

Under the Other section is my Control Panel and also my BlackBerry device (no drive letter). I can click that and access the media card as mass storage.

Hope this helps.