Paranoid about security on formatted computer?

Hi,

My computer recently died, and I handed it over to a tech guy to format it, save my data, and reinstall Windows

I'm really paranoid about security, so I'm just wondering is it possible that he could have embedded some sort of hidden malware/trojan that could give him access to my computer? Dont have any real reason for suspicions, but I just want to be sure

Avast, Spybot and Malwarebytes all come up clean

is there anything else I can/should be doing to ensure my system is secure? Would using OTL and posting that log be helpful?

Thanks alot - really appreciate any help

Gavin