Three Sheds, probably the best option is to use a later version of Windows which might handle memory better (it's not always the fault of the application programmer and Unix, etc. did not earn their reputation for being more stable than Windows because only Windows application programmers made mistakes with things like malloc). Also, be on the lookout for a rogue application. If you are frequently hitting problems, it is likely to be that. It's imossible for me to quantify but with my usage of Win 98SE or 2000 pro, I'd expect my system to run for a week.

It's a long while ago for me but from what I remember, one of the weaknesses of some versions of Windows was that for things like GUI (was it - I think they grouped them under "resources", they allocated something like a global 64K (maybe it was more) for all apps so there wasn't really a lot of space for some purposes even if you had 128Mb of memory.