this really ought to be a string of expletives, but I can't bring myself to publish that

I knew today was going to be kind of sucky (appointment with meds person, meeting with strangers), but it has already exceeded my expectations by far...
My meds person has given up on the wellbutrin, so now I'm supposed to taper off it, while she tries to think of some other plan.
I got to the meeting at a building I have no access to and realised my cell phone was dead so I couldn't call my advisor to come and let me in. Instead I had to sneak in with someone who happened to be passing, then try to remember the strangers' names to tell the receptionist who I was looking for. Then, it turned out I was supposed to give the same talk I gave twice last week again, this time to only 2 people (smaller, more personal groups are much scarier than big, anonymous ones). Now I'm supposed to be reanalysing everything all over again.
So, I went looking for a file on my computer and found a text file called "I read your email". I was hoping it was a draft of an email I'd written that got named by the first few words, but no, it's as creepy and horrifying as it sounds. Here's the whole file:

Look, your files are viewable by everyone.
I just turned on my pc and it magically mapped your drives. maybe you want to take a look at that?

The folder the file was in was set to be shared (something I'm pretty sure I didn't do). I made it private again, but I have no idea what else I need to do to prevent people from seeing my files. Does anyone know how I can check if they're still visible? Would that one shared folder let someone into my whole computer? I do have a firewall, but it turns itself off occasionally. Oh, and the worst thing? The file was created in January...
I'm hoping it was some random person on the lab network, who doesn't know me and is not interested in stalking me. I'm pretty sure my housemates would actually tell me, rather than leaving a secret message and I think my labmates would do the same.