Windows XP Question.

I have a relatively new system (3700+ CPU, 1GB PC3200 RAM, GE6600GT video card, Asus A8N motherboard). I have applied various tweaks to my system, but always make sure they work OK, or I restore. However, something has happened within the past few weeks, in that when I exit out from a program, then go back into it, it's as if there are no pieces left in memory from the previous session. I used to be able to go into Outlook for mail, then onto the web (neither app taking much in the way of resources), then when going back to Outlook it would load in a snap- presumably from parts of it (if not all) still residing in memory. Now I can't do that, and returning to Outlook takes as long to load up as it does after a boot.I can't say that I have noticed a slowdown in performance when actually in a session (including FS9) so far, however any tools that you can recommend to research this would be very welcome.Any ideas? Any would be much appreciated.Thanks- Bruce.