george, I don't wish to appear rude but surely if you're planning to become a private investigator you'd already know the answer to this question? To be truthful if I bought a computer and it still had stuff on the hard drive from the previous owner, curiosity would get the better of me and I'd have a "butchers" but certainly not with the intention of doing anything illegal with the information. A few years ago there was a case in my local paper about a guy buying a secondhand computer that still had stuff on the hard drive and when the guy checked through it he found it full of child porn! It was actually a local computer shop that had sold the computer who said they normally made sure the hard drives were wiped clean before they resold them but this one must have "slipped through the net". What surprises me even more is that the computer shop didn't check the computer when they bought it off whoever had sold it to them!
But getting back to your question, I can understand people's suspicions when you post one question about the legality of looking through someone else's hard drive and then, within half an hour, post another question about the legality of using an alias. If, on the other hand, you're genuine and these two questions are purely coincidental, I apologize. In regard to your other question: As far as I know it's ok to use an alias as long as it's not for fraudulent purposes.