This is a pretty commonly reported problem, and you might find some insight into the situation in this Joomla! Forum post:
I can confirm that many times uploading the huge number of files in a Joomla install can be problematic - if your ftp client burps at all, you are likely to have some incomplete files or some files not properly uploaded.
I avoid this problem by unpacking the Joomla! tarball directly on the server (plus it’s much quicker).
Additionally, you can use DreamHost’s “one-click” installer for Joomla! 1.5 - which is just about as painless as it gets.