Free computer how can you be protect online ?

Hi to all,

A friend of me just give me
1- pc
and 1- laptop

I'm not sure if they are good one new ore stolen i do not want to get troubles or traces for others use them how can i know and what can i do
(change hard disk, serial xp, or others numbers can see me please)

Have any idea please (people tell em in pc change hard disk buy a new windows xp license) in laptop impossible get vista and a serial number can do nothing ??? thrue false please

thanks a lot