You reinstalled windows to format your usb? try this....
Plug in your usb
Go to start>programs>accessories>system tools and run command prompt as administrator.
Type: diskpart (hit enter)
Type: list disk (hit enter)
Here you will be able to see your usb and the disk number
Say in this example your usb is disk 1
type: select disk 1 (hit enter)
type: clean (hit enter)
type: create primary partition (hit enter)
type select partition1 (hit enter)
type: active (hit enter)
type format FS=NTFS (hit enter)
type assign (hit enter)
then exit.
your usb should be good to go. Sometimes the format does take a little while.