What Does a Boy Think when he sees a Girl as Topic seems it may be simple but it is not .it is going to be critical but i will keep it simple
So there are 2 People:-
imagine in a empty room a girl sits and gazing through windows and then a boy opens the door and says "There You are" .This type are mostly Confident ,Caring and Carefree and Other boy Opens door and says "I am here".These type are mostly have Pride,Power,Confident and Attitude ...

Difference People think different depends upon the growth Environment
Common things that boys think when they see a Girl
1.OMG she is Hot
2.She has got any Shape it may look cheesy but this truth
3.She got Style or Not
4.She is Alone or got any Boy friend
Once this are not accurate and there is more that boys think when he sees a girl but i wont be telling it is upto Readers to Imagine

NOT ALL BOYS THINK LIKE THIS

More articles who may be of your interest:
http://www.living.oneindia.in/men/unsaid-things-about-boys.html
http://in.answers.yahoo.com/question/index?qid=20090227031938AAitpzV