I hve just come back to Chaing Mai after a year and a half away and am staying at the same guest house as before, the set up is still the same but things seem to be differant, maybe as I can now speak a little thai I know they are talking about me (good or bad is hard to tell). Also there was a robbery the night I arrived. I will go to Pai soon

Due to this I have bought a kensington security lock and attach my laptop to my welded metal bed. How secure are these locks?

Not very, but better than nothing. Do a search, there is a good topic somewhere about this.

only as good as the plastic case mate - maybe you should consider a new guesthouse

I have used these cable locks for years. My thoughts are that unless they come with cutting equipment they cannot take the laptop. However I am sure some people would smash the latpop just to because they could not take it. I think really these stop the amature theif not the professional :-)

What ever you do always keep a backup and insurance :-)