If you have edited any of your core files (the PHP files) using a HTML editor, MSWord, notepad - or an editor that formats text, you are likely to corrupt the file.
Now, I have no idea if you have done this, but if you have you should check all the files you edited for errors.
The first thing you must do is download a good text editor - CRIMSON editor is a good one and it's free on the WWW. Google for Crimson Editor.
Next, download and install WINMERGE.
You will use Winmerge to COMPARE your edited copies with the originals. Winmerge will find differences in the two files and highlight them for you - making corrections easier.
You should FTP any changed files back to your hard drive, and do edits there. Then make sure you FTP the edited files to your custom (over-rides) template.