Undetectable Hack on My Pc

hi all hackers

my windows pc has been hacked with files and programs being added and removed and some skype conversations with friends i didn't have

but Ive paid a couple of experts to examine them, but neither have found how they have been hacked

any advise on how this is possible ?

is it just a hard to detect back door ? if so what is the hardest to detect ?

thanks for any advise on how this is possible