The lock icon?

It should be nothing major unless you are planning to share them over the network or something similar. It only means that you are the only one allowed to edit them.

As for .mp3, .jpeg, you can't edit those types of files (unless you count editing .jpeg by Paint).

PS: Do you have Windows Seven? Your specs say you have Vista, but the display looks like Windows Seven.

Oh, I haven't updated the specs. Yes, I have Windows 7.

I suppose you can't edit mp3s directly (you can open them in Audacity or something though), so it makes sense... Is there a way to fix the fact that Firefox downloads everything as only editable by me? I don't mind it as much as the fact that they make the files look locked...

it might be that the professor has made these powerpoint presentations with specific permissions set. when you open them, does it say 'read only' at the top? try checking the permissions of the file in powerpoint, see if you can see a clue as to why that is happening.

It's not read-only, no. Dunno. I saved one in Linux and transferred it to Windows and it showed no lock, but then when I redownloaded it in Windows it did have a lock.

Nope.

its to do with user permissions within windows. your user has access to the file, but any other user logged in in their account doesnt (even if your the only user on your computer).

1) Right click offending folder, go to properties, Sharing, Share,
2) Select "Everyone" in the drop list, click Add, then in the permission level drop list set to Read/Write.
3) Click Share, then done.
4) Click Advanced Sharing.
5) Uncheck "Share this folder" then OK and CLose.