Of course, encryption is as good os the person who wrote the software you are using... Trust Allah, but tie your camel as the proverb goes
Especially for Excel, I woudn't trust its encryption as far as I can throw it!
pwsafe is free and open source and its implementation of encryption routines is well documented and tested by peers. It can be downloaded at http://passwordsafe.sourceforge.net/
There you can store all your password safely... but to tie your camel why not use code word as well... An old trick of spies is to choose a book in your library. Picks words as your password, concatenating them. When you need to write the password you choosed, write the page number, line # and word # in that line. This {132,4,2},{422,12,4} is your password. Then encrypt it. It is useless to anyone who could decrypt it unless they know which book you used.
And always clean you computer disk regularly. Try and wipe the deleted portion of your disk if you can using specialized software.
If you use word to write compromizing letters use safe as. this will clear the undo buffer of the doc which could be analyzed. Converting to pdf is better.
This is of course the tip of the iceberg when you really want to be secure, the art gets better and the tricks more and more amusing
Try http://www.nsa.gov/snac/index.html for guidelines
(of course remember that whatever YOU do THEY can read and decrypt all. you can bet your ass on it!)
Cheers