if u r using windows 7 iso image and copying it to usb using wintoflash,this problem is coming,even i faced it too,i have a hp flash,i simply copy paste iso files in flash,n go for booting,but my friend has kingston flash,but this method does not work,he needs to use wintoflash.
there are 2 ways.
1.go to tasks,and then copy the setup.it shall work.
2.use magic disk,to load image in some virtual drive,then format usb in my computer in fat32 format,then come to wintoflash,n copy(it will again format during this)...and even if this does not work,then go to BIOS,set ur boot preference to usb and even if this does not work,see in BIOS if u have AHCI/IDE option,change it to IDE temporarily,n then try.
or even if still this all does not work,tell me,i have a very simple way to make bootable windows7 dvd disk.