Just follow the steps as given below :-
Insert your USB drive into your system and wait for it to be recognized..
STEP 1: Go to Start >Control Panel >Administrative Tools >Computer Management >Disk Management >
You can go directly by typing the command in run: diskmgmt.msc
Then right-click the partition whose name you want to change (click in the white area just below the word “Volume”) and select “change drive letter and paths.”
From here you can re-assign the drive letter it was given to “A”
NOTE:Drive letter of Pendrive must be “A”
STEP 2:- Start->Run->syskey
click on “ok”
Syskey launched: Click “Update”
Choose “Store Startup key on floppy disk” and click “OK”
You’ll be prompted to enter your diskette. Make sure your USB drive is inserted and writable.
Restart and have fun. Don’t lose your USB disk! Also, to revert this, you can run syskey again and choose to store it locally instead of “on a floppy disk”.