OMG - are you kidding me?????? DON"T listen to "everyone around here" -- contact the police, tell them everything you've said here, make sure to document EVERYTHING and DO NOT have contact with this guy. I can only assume you didn't talk to your boss either........ as he came to the office and was LET IN.!!!! You need to talk to your boss, let him know what is going on and that it's HIS responsibility to protect EVERYONE at the workplace - he should've NEVER been let in. Also, HOW did he know most everyone was not in the office that day????? Is he WATCHING your office?? YOU need to take steps to not only protect yourself but your co-workers. This guy sounds like he's not stable and could at any point go off.
I almost think from your post that you don't really take this seriously as you've not contacted the police or told your boss - your IT person IS NOT your boss - I assume. There are so many women who get killed because of people like this.......... do something to protect yourself and your co-workers - from everything you've posted it's very clear he's disturbed and it's time you do something about it.
** Kimmar - Hey Sweetie......... WOW - hopefully your nut doesn't contact you anymore!!!! YIKES!!!