I am entering Thailand on a 60 day tourist visa but i want to go to Cambodia for a week and then return to thailand. how will this effect my visa? will it cancel out the 60 days and will i not be able to extend my visa at the end of the 60 days. Please can anybody advise me.

It will cancel your visa entry unless you obtain a re-entry permit prior to exit of Thailand - that will cost you 1,000 baht and a current photograph.

Thanks. Can i get the re-entry permit from the immigration office at Jomtien?

Yes you can.