2000/XP was always extremely aggressive about allocating memory for cache. It would very often swap stuff out to increase available cache, which made the apps you did not use for just a few seconds completely unresponsive.
Specifically, minimized applications got their working set trimmed. You could see this in Task Manager: it's Memory Usage column was actually Working Set (I think they're more explicitly labelled these days) and it frequently went from many MB down to a few KB once you minimized the app. Working set is the set of pages, currently in memory, that the app has used "recently", and are thus less likely to be paged to disk. Shrinking the WS makes pages more likely to be paged out.