I have a 16gig flash drive when I plug it in a new drive letter appears but instead of a flash drive recognises it as a cd rom drive how do I fix problem

With the drive connected, get into Device Manager (Start | Run | "devmgmt.msc" | OK). Under "Disk Drives", delete the entry corresponding to the 16 gig flash drive. Under "Universal Serial Bus Controllers", delete the USB Root Hub that corresponds to this drive (right-click on each Root Hub) and see how many ports are available under "Power". (Unplug the drive and then look through the list again to see how many ports are available. The port that corresponds to the drive will change).

Remove the drive and reinsert it or click on the "Action | Scan For Hardware Changes" icon or menu item.