Thanks for the replys.
An update to my issu :
On my Windows 7 running on Dell Inspiron 1520, In Device manager i saw some yellow icons in Universal Serial Bus controllers, and by disabling and reanablin them, the device showed up in my computer, it demanded a fix, i performed and now i can use it with no problems.
But on my XP running on a not much new PC, the device is recognised in device manager, even the icon of safely remove it appears in task bar and even i can open its contents using Tune Up Disk Explorer, but just it does not show up in My Computer.
I wonder that whatever the hardware of my PC is, it had no problems before with disc external hard but just recently.
Thanks for the help !