Well, as you know, there's more to a computer slowing down than just having extra junk on it. Software gets larger and more functional all the time, relying on the advances in the underlying computer technology to take up the slack with more speed, more memory, faster disks, faster networking, faster internet, everything more and faster.
To speed up my daughter's older laptop which had gotten very slow (and the hard disk finally broke when I started to go through my normal ways of speeding computers up), I did a couple of things. First off, I got a new, bigger, faster hard disk, and max'd out the memory capacity *smile*. Then I re-installed the OS (which I had to do anyways). And switched to just using MS Security Essentials and the Windows firewall from the bloated firewall/antivirus/antimalware suite that she was using. And advised her to not install anything on it unless she was sure that she absolutely had to have it. Well, that really speeded it up a whole lot, and made it much more useful to her for basic stuff like surfing the internet, email, IM, Facebook, etc., all the lightweight simple stuff.
Re-installing is really something to consider to speed up a computer, it is the ultimate cleanup of everything, the registry, the disk, nothing like a clean start to speed up a computer. And then just install what you absolutely need. This would also be a good time to get a newer, faster, larger hard disk and more memory to give your old computer a chance to be faster. Personally, I'm giving a hard look at the new hybrid disks with mixed SSD and rotating disk technology, the price and speed of those is getting very attractive lately.
And of course, there's all the standard speedup stuff that's been mentioned before, like making sure that you're not starting any more software at boot time than you absolutely need (msconfig), and removing software and files and cleaning things up generally, defragmenting your disk, and so on.