After having my computer formatted, I reinstalled u-torrent and tried downloading. Some torrents worked fine, but close to half would come up with error messages like "Cannot access source file". I tried reinstalling, then tried an earlier version, but it didn't help.

So I tried Azarues, but am experiencing the same problems, error reports including "disk write error' and "file close fails"

Any tips or suggestions?

Google (or check out the Utorrent FAQ/forum) Azerus or Utorrent disk write errors. Plenty of people reporting all sorts of potential issues: bad disk sectors, controller issues, other, which might apply to your situation?

Check to make sure that the folder you are saving the torrents to have full/permissions it might be that the folder is read only

Some torrents have trackers too that require registration before being able to download. Just a thought.