Spellcheck for O.E.2.1 - overwrite files??

I have downloaded Spell Check for O.E. 2.1 and have it on my desktop. When I went to install it onto my 'puter, a message saying "Some of the files are write protected. Do you want to over write them anyway?" came up
What does this mean? Should I go ahead and install the programme? Is it a good programme to use? Any ideas would be appreciated.
I want a spell check in Outlook Express - using XP and Works suite (therefore no spell check available)
Thanks for any help