Over the past few days, when I try to download files, I often get a page of scrambled text/code etc and do not get the appropriate download.

Also, day to day, the permissions on my files changes and I should be the only one on the computer.

Firefox will not download, though it has for the first month of Vista. I have run chkdsk, diagnostics, system restore etc.

Suggestions?

Hi there and welcome to PC Help Forum

Is it only for certain filetypes? Did you consider resetting the associations to their defaults?

Here are some specific file type fixes: File association fixes for Windows Vista

..and here's a great walk thru (with pics) for changing the associations: How do I... Change file extension associations in Windows Vista?