I am not able to use my usb pen drive in My Computer.

I have windows XP sp 2 installed.
Whenever I plug my pen drive, it directly shows "safely remove" icon in the taskbar.

My Computer does not show USB drive letter.

If I want to access the Pen drive , then I always need to do this :>>

>right click My Computer
>click Manage
>click disk management
>click partition which is not showing drive letter.
>assign drive letter to it.
>then right click on it... click open

This is how I can access pen drive.

I am getting this problem since from when I updated my windows xp sp1 to sp2.

How to solve this problem ?