Strange USB flash drive problem...

Hi everyone. I have a kingston data traveler 256 mB. Previously I had an unauthorized copy of windows XP pro ed runing on my pc. Everything was working fine. Then I decided to purchase an original win xp pro disk, installed it as an upgrade into my system and everything seemed to be OK. When I inserted my flash drive, surprise! It was detected, but doesn't appear on windows explorer, nor my pc. On the device manager, it's listed on storage devices, but I can't access to it because it simply doesn't appear on the explorer! I've tried changing manually to drive f:, g: etc, but none of them exixsts. The flas drive works fine on any other PC. More strage yet, all USB devices I have plugged in (bluetooth adaptors, web cams, game pads, etc) are fully working! So I decided to try with another flash drive and the same happens: It's detected but there is no way to access it from thye explorer! And the same happens with any storage decive such as digital cameras, all of them are recognized and shown in the device manager, but there is no way to acces them because there is no icon or path on windows explorer or my pc. What could be possibly going on? I've tried on every USB port in my PC, uninstalled and reinstalled USB and hub drivers, and the same is going on.
Please help.

Is your XP SP2'd?

Rather than opening device manager - open computer management (Right-click My Computer - MANAGE) and highlight **** management ... can you see your devices there?

If so - try mounting them.

Ok

Yes, it's SP2.
And thanks, that solution worked wonderfully. Now I've made a backup of my data. I still don't understand why this happens, but anyway I can open it from there now.
Thank you again!