i recommend firefox and google desktop wholeheartedly. at least from my experience, google desktop is very easy on system resources, and much better than standard search (i've heard yahoo is good as well). picasa is also very good if you need an image organizer which does a few useful enhancements.
nothing else stands out as important. obviously get acrobat reader if you don't have acrobat.
edit: under additional software, trillian is listed and is decent. although not a hot deal, this can be useful to some if they don't want to search for each individual program, which might not be the right audience here, but someone will benefit.