"Junk" on your hard drive does not affect the speed of your computer in any fashion, unless there's so much junk that you've consumed more than 90% or so of the hard drive space.
Installed programs that run in the background (99% of apps do not do this), like Launch Bar, Quicksilver, or other "always-on, never quit" applications can slow a system down, so check your "Login Items" for programs that start up at log in that you may not need anymore.
I highly recommend upgrading the actual computer itself, as in adding more RAM or upgrading the hard drive to a larger, faster drive. Those two things will get you the most speed boost of all, while allowing you to continue to accumulate junk...
Of course, a good, old-fashioned re-install of Mac OS X always gives a good
perceived speed boost (although more than likely the system is not running perceptably faster) because of the placebo effect of thinking the system is "cleaner." You may also look into upgrading to Mac OS X 10.3 "Panther" or even Mac OS X 10.4 "Tiger" if your system is supported.