the nice thing about using msconfig is that you can always go back and change things again. Clearing the checkmarks doesn't delete anything from your system, just takes it off the startup thing. Too many software packages install all their junk in startup without even asking you, or without telling you that you don't need to have that activated for the program to work fine.

"older, outdated machines are horribly slow..." GUEST is talking nonsense. Machines which aren't configured properly are horribly slow. There's nothing wrong with a PC 2 or 3 years old if you follow proper maintenance. Just like a car. RAM upgrade is good to have though. Specially on Win 2K or XP.   

If your C drive is 95% full, more than just defragging performance is affected. You simply won't have enough headroom to perform a lot of functions efficiently. Gotta move stuff out of there. Remember, that's where your operating system is--and while the swap file doesn't have to be huge, you need to leave some room for it.