All the programs mentioned are really good and extend the life of the installation by a lot, but there will come a time when it's just too old and clunky and needs to be reinstalled.
I don't agree. I haven't had to reinstall one system of my own due to slowdowns. This goes all the way back to DOS (yea I still have one machine running that). Never had to reinstall any win OS due to slowdowns. Several times due to hardware failure but that is it and those reinstall only went back 3 months at maximum.
Now other people tend to screw their systems up weekly. It's all in the user.
Eventually though all systems will become virtually unusable just due to windows updates. It's almost unbearable to run XP on a P2 or P3 anymore with just a base install, updates to SP3 and basic firewall/anti virus. Just not enough memory on those older systems and they can't be upgraded due to hardware limitations.
Agree 100%. The only windows OS that I have ever had to reinstall on a regular basis was Window ME. I too have never reinstalled my own machine because of slowdowns, nor have I had to do this to any machine in my house. On the other hand, at work, I routinely reinstall windows as a virus removal method (its faster to remove the hard drive, attach it to another machine (which virus and malware scans every file copied), copy the customer data off to a temporary drive (or DVDs and then we can sell it as a backup as well

), put the drive back into the customer computer, reinstall windows, copy the customer data back, all of which can be done in about 4 hours (depending on how much data needs to be moved) and we know that the virus has been completely eliminated).
Windows ME was never reinstalled because of slowdowns though, it was always because the the OS would start glitching after about 2-3 months. The "reinstall" was done using Norton Ghost, so we would be ready to go in about 30-45 minutes.
Most of the software that "optimizes" you computer is snake oil at best, and destroys the OS's stability (sometimes to the point of making the computer unbootable). The only "cleaning" tool that I use is CCleaner and mostly just to clean up the temp folder at that. But even its registry cleaner (which does not "defrag" the registry) only removes things that it actually knows are safe to remove.