If your pen-drive is not recognized in Windows 7 then you will need to manually format it. You can do this by entering Computer Management found in the Administration Tools in your Control panel.
In case you want a faster access to the tools please go to Start > Run > type "compmgmt.msc" (without the quotes) and then press Enter.
In the windows click on Disk Management and wait for the list to generate. On the right side of the window, locate the pen-drive (you can identify it by its size) right click it and choose Format. Upon formatting you will be asked to provide a letter for the newly formatted drive. Choose one available and then access the drive from My Computer.