uhi this is bcos u havent the requires usb drivers installed on to ur system
two steps one easy thje other a bit longer
just copy and paste the blow given link on ur IE address bar
it will direct u to a site where the required driver is available
download the exe file onto ur system
once done run the exe file
once finished restart ur comp
now plug in ur usb
To open Device Manager, either run Devmgmt.msc at a command prompt,
or follow these steps:
Click Start, right-click My Computer, and then click Manage.
Expand System Tools, click Device Manager, and then click Devices by type on the View menu.
Right-click USB 2.0 EHCI Host controller, and then click Update Driver.
You can find the USB Host controller entry in either of the following locations:
Universal Serial Bus Controllers.
Note In the Other Devices location, the USB Host controller entry is labeled Universal Serial Bus controller with a yellow exclamation mark.
Click Install the software automatically (Recommended), and then click Next.
Click Finish when installation is finished.
now plu in ur USB ;)