At times, I've installed knoppix to HD and just reinstalled when the next version came out. This isn't a good idea on any PC that needs to be secure, but it is unbelievably easy to do and works well enough.

Any current OS/distro installed on an old PC is likely to run slowly, but probably not as slowly once installed than if it's run from within RAM.

How much RAM is currently in your system, by the way?