How to change the user name on your laptop systems information?

I've bought a HP Pavilion laptop of a work colleague and I want to change the user name on the system to my name. I've managed to change it on the systems properties tab where it says registered to... but file path names etc still keep coming up with the persons name I bought it off. when you go it to run from your start button and type in winmsd.exe it brings up the systems information eg. OS name, version, manufacturer and the actual system name consists of two letters and the rest numbers, further down the box is user name starting of with the systems name then forward slash and then the owners name eg. PC12345678910/joebloggs, how do I change that name its really bugging me.How to change the user name on your laptop systems information?you could do a system recovery, he has probably registered it with microsoft in his name? Have you got recovery discs? Or reformat the drive? You can also try admin tools, PC management in control panel?