My USB has files with no pictures of where the data came from. I didn't even create those files. I tried to delete them but it said "Cannot delete file: Cannot read from the source file or disk" Does anyone know how to delete them?

If not mistaken you could use process explorer to see what process is using the file and then kill it.

As for the exact details on what options to click; I'm unsure.

Also, sometimes when I try to save something on to my USB it says "Word cannot save or create this file. Make sure that the disk you want to save the file on is not full, write-protected, or damaged" I checked and I have a lot more megabytes left in the USB.

Can anyone help me here?

Try using Unlocker to delete the files.

Are you sure the USB device doesn't have an external lock like a little switch that is in the "lock" position?

i think that happened to my usb drive the other month, i couldn't right to it or anything it appeard full there were jibberish files all over it. i tried unlocker no success, so i ran a file recovery app on it and reformatted it i had no other choice
and some of my data was dammaged it sucked but it whent away after i re-formatted it.
i think it happened to mine when i pulled it out while the mac i was using was reading from it..

Some USB sticks come with files that cannot be deleted because they are relevant to it's use. IIRC they don't have file extensions and so would not have a programs icon. About having space left, it works the same as with hard drives, if you buy a certain MB/GB you always get less