Helping people with computers... one answer at a time.
If your computer is not physically secure, it is very possible for someone to install something unwanted on your machine even when you're not logged in.
Before I raise heck in the house, would you please answer a question? We have several computers in our home. Recently, I have seen "spy pc 7.0 Quick start guide" in the home. I don't mind if the owner of this booklet uses it on his/her machine; but not on anyone else's. Can this be installed on other personal computer (which are usually password locked)?
The short answer? I think you're right to be concerned.
There are a couple of things about this scenario that bother me, and should probably bother you.
"Usually password locked". I'm assuming that by "password locked" you mean either that you are not logged in to the machine, or that a password protected screen saver is running. Either way ... usually? That in and of itself is an open door. If you're away from the machine for any length of time while you are logged in, the door is wide open.
But that's not really the worst news.
What many people fail to think about is this:
If it's not physically secure, then it's not secure.
Think about it. Anyone walking by your machine, your "password locked" machine, with a boot floppy or CD-ROM could reboot the machine (by pulling the plug, if needed), boot from their floppy or CD-ROM, and get access to everything on your machine. And that includes installing spyware, reading your data, messing up your files,and whatever else they might think of.
So if you have a concern, if your machine is physically located such that untrusted people can have access to it - you are vulnerable.
I'd lock the room your computer's in.
No, actually, I take that back. I'd get more trustworthy roommates.