You been wardriven
I'd bet on it, someone's hacked your wireless net. Or simply logged on to it if you didn't reset the password to a ten digit scramble.
Dead giveway;
also I have found documents on my computer, nothing I ever wrote or downloaded, I have a multi megabyte document on fuel injectors that I KNOW I never would have downloaded...
Anytime you're getting files or docs in toto on your system such as that, someones been using it besides you.
Do what ET and the others said, then lock your network down as tight as you can, including the scrambled password, using every symbol and character the software allows.