unable to boot from USB

Dear Team,
I am using windows 7 and now want to install windows XP. now when i try to open from my pen drive a blue color window appear with a message default and an the bottom of that window 2 messages are showing : 1) press [tab] to edit option.
2) automatic boot in 10 seconds..... and 2nd option continuously coming again n again.
when i hit Esc button then "Boot: " message comes and i can enter anything on here but every time could not find kernel image message showing.
when hit [tab] ket then ">ubnkern initre=/ubninit " message comes.
i m stuck. pleaase help me as soon as possible.
thanks a lot in advance.