you're asking for 2 different things:
1) cleaning up the junk
2) speeding up your computer
for cleaning out junk - i assume you're using a PC - try
ccleaner
no matter what software you use, you have to be wary.
backup everything important before starting the clean-up process.
especially if you clean up the registry.
however, there is no guarantee that this will really speed things up.
the longer you have your computer, the more stuff you install, the more junk builds up.
cleaning out junk can help speed-up your computer. for example, if the cleaner has flushed out the internet browser cache, then your browser should load and run a bit quicker.
however, to truly speed up your computer you'll need to get a faster processor, quicker RAM etc.
the biggest problem in terms of slowing your computer is the hard drive. you'll either have to get a RAID-type solution or get SSD.
i constantly run cleaners, defrag the drives, defrag the registry, keep software and drivers up-to-date etc and yet... sometimes the computer is still sluggish.
my computer did run a tad bit faster when i had it in a RAID0 configuration. it was not significantly faster but you could see the difference.
i haven't tried SSD so cannot comment on that.
if you can, get 16gb (or more) of RAM and make a half of that a virtual hard drive. then use that as your pagefile and other temporary work area. for example, use that to work on authoring dvds. you will get a much faster throughput.
however, if the power goes off even for a second, you'll lose some/all of your data. so only use this option if you're willing to that that slight risk and that you can always go back to the original data/files from your real hard drives.
ok, hope all that makes sense!