I have a problem with my MacBook Air. Help!

Hi. I have a MacBook Air. 1.86 Ghz Inte Core 2 Dou, 2GB DDR3
The problem is: It overheats. Things go slowley. Any suggestions?

My brother used to have this mac before me, how do i change the "username"?
(under desktop on places)

Do i need to reinstall to o.s.? And how can i do that on a Macbook without CDroom?

Thanks.

Just go into "system preferences" then go into "accounts". should be pretty self explanatory at that point.