Normally I don't have problems with my ram. When I do, that's when a good format/reinstall of Windows is in order and makes everything run nice and smooth (including 45 second boot times!). Otherwise, I use Maxmem by AnalogX (
www.analogx.com). The bad part about memory cleaners (all of them!) is that they actually _increase_ disk usage to some degree. That is, you dump out everything in the RAM, and then have to wait while your computer reloads everything it was actually using, while the dead weight is (hopefully) gone until the program that caused the mem usage or mem leak is loaded/used again.
As for OSes, I use Win98 first edition on my main computer (I don't have SE and I'm too damn cheap to get it). WinXP has a bad habit of consuming a large portion of RAM (I'm pretty sure this is why most gaming articles on the issue I've read denounce XP and cite benchmark tests to back it up).
Now, I remember back in the day where we had 64 megs of RAM and we were happy. Nowdays you need 256 before your computer can crash without giving you an out of memory error for trying to load the blue screen of death!