More Ram is always needed. QXP is a Ram hog so you should have at the very least 512mbs.
Another thing to keep in mind is the size of images you have in a given doc. For the small amount of ram you currently possess, images over 1mb will begin to slow QXP down during a scrolling screen draw.
Another thing that will slow it down is your over-all document size. Try to keep your docs under 20 pages, especially when there a lot of graphics/images.
And lastly, Ram is what your applications use while open. An open application, even when idle, uses Ram. If there haven't been any QXP performance issues while running other apps, then the current doc(s) you are working may be larger than normal.
Of course I'm assuming that there is nothing wrong with your hardware. You should give Norton a run. After it makes repairs, use it to defrag your HD. That will also make your system more peppy, especially given the fact that your HD is getting maxed out on storage capacity.