i won’t claim that i am a computer expert, but I know much.
hacking into someone’s PC can be very easy or very hard
did gave this man the chance to sit in front of your computer, or you opened any file that he sent to you?
* if yes, then let me tell you that even a 10 years old child can hack your computer easly if he could sit in front of your pc, or if he could make you open a file he sent to you.

* if no, then maybe he could get into your computer without your knowladge?
do you put password on your pc? I don’t think do

Anyway the BEST soultion is simple, cost no money, but you will loose everything you have in your PC
if that’s ok with you, simply format your entire HD, if you could ERASE it that would be the safest solution

note: u may think, “why don’t i make a backup of my HD before i erase it?
the answer is, making backup will backup all your files including the infected ones
and since you can’t tell which files are infected and which are not, you have to erase all of them

If you need more information about this, feel free to contact me

There is another solution but it is not guranteened to fix your problem, but most likely it will.
this solution is buying an expensive anti-virus and anti-spyware scan your computer very well using them and delete any infected files

last solution is by simply installing another SAFE operating system like LINUX
even if this guy an expert in windows, I am sure he know nothing about linux
Linux is the system of my choice, i’ve completely forget everything about viruses and spywares, open any file i download without being scared that it could be a virus
but linux is alot more different than windows, you will have to learn from the begining

3 choices, choose what you feel better

yep