A valuable indication of too little RAM is a lot of hard drive activity when you're not initially opening a program, opening a modest size document (word processor, pocket camera image etc) or saving a document. Even with adequate RAM you might have disk activity with things like locally stored video, images from high end cameras or sophisticated programs like editors a professional photgrapher would use.
With Windows XP Windows Task Manager can put a nice little CPU loading icon in the task bar's notification area, although many systems sold with Windows XP might be using nearly the full CPU capacity playing video. Streaming video from an online source would require more resources than playing a video from a local source like a hard drive. An unencrypted file generated from your own video camera might also require fewer resources than an encrypted video like a commercially produced DVD.
I haven't tried it yet, but a local acquaintance recently
recommend a program called WhatInStartup for reviewing and controlling what gets loaded during Windows XP start up http://www.nirsoft.net/utils/what_run_in_startup.html
. The msconfig program that comes with Windows can be usually be used for the same purpose, but you might have to Google some file names to identify what they are and what purpose they serve.
I often find that it helps to close other programs when I use a browser to play streaming video from sites like YouTube, although the issues I have are more like hesitating images or audio out of sync with video. The mail program I use sometimes uses a lot of CPU cycles and it usually helps if I close it when I visit YouTube.