Computer Lockdown

I know this probably isn't the best place for this question but I trust everyone here so I thought I should ask here. I really don't mind people using my stuff when they ask, but with the Alienware lappy I'm super cautious. Lately my roomie has been on it a whole lot more than me and it started acting weird, mostly with how it came out of screensaver and virtually all that should have been in the taskbar was missingm, I had the clock and AVG not even a battery monitor. So I passworded my account, and made it so when it resumed from screensaver that it would prompt again. Everything goes "normal" ie it's still acting weird with the mentioned things but not degrading. So I'm trying to figure out how to get my taskbar stuff back up and restart for something and login, I then get a call so I leave the room so that I can get better reception. When I get back I notice that the screensaver isn't up (so I think crap I should have locked it after logging in) but then the worst of it happens. I can't log online, I notice the wireless adapter is off, so I re-enable it; blue screen, it reboots itself 4 times and when it gets to windows it bluescreens and reboots again. Every so often I could get windows up but turning the card back on kills it all over again.

So I backup all my crap and reformat it (which really pisses me off because I NEVER have to reformat due to problems, only maintenance) As soon as it's up and running I get AVG and ZoneAlarm on and figure I'll worry about the rest during Thanksgiving break. Oh and yes of course I password my account with a ridiculously long password and it also contains numbers. So what do I find today when I wake up? All 30 some odd astronomy papers I was researching online (I'm an astro major) all
closed. I come back today from TAs office hours and guess who's on asking: "You know your computer says it can't open an Office 2006 file?" Of course not it doesn't have office anymore it's reformatted! Now how the hell do I stop this from happening?! Damn sorry for the rant but it frustrates me that you come to college without a computer and then my other roomie gives him his old laptop and he's still on my computer!

Yeah that's generally what I do when I leave, but I also have the screensaver do it, and I double checked that it was locked last night but he was still on it.

Yeah I don't know, but the last time he was on it I watched him and he didn't log out or anything and my account was what was logged in, and nothing shows up under user accounts but mine. But my friend said in Safe Mode the admin account shows up and it's not password protected, so I'll check that one out too.

I already have, and the fact that my other roomie gave him his old computer makes it so he has no need to get on here, but yet again, when I logged in; someone left the network neighborhood open and was going through my networked external drive...