What Could Be Causing My Laptop to Lag for Everything?

Basic Specs:

2010 Asus Laptop

124 GB free of 283 GB
0% disk fragmentation
2.2 GHz Dual Core Pentium CPU
3 GB of RAM
64 Bit Operating System
50-65% RAM usage, on average, per the Task Manager

Open Programs (currently, as a good example):
MS Office (Word, Excel)
Adobe Acrobat Reader (15 documents)
MS Notepad (3 notepad files)
Windows Photo Viewer (3 images)
Firefox (3 tabs [Gmail, CNN.com and this forum])
AND THATS IT

Yet everything lags, especially if I haven't clicked on an app for a while. Then, it will take 15-20 seconds of the mouse pointer spinning and the app not responding before anything happens. For example, if I am using the web browser for some time, and then click over to view a PDF file, it will lag bad for around 15 seconds before the taskbar displays the open Acrobat files to select from, and then, once selected, the Acrobat document will appear piecemeal on the screen and, even then, scrolling will be very staggered for another 30 seconds or so, till it finally settles into some degree of useful smoothness. If I restore another Acrobat document at this time, there is less lag since, I guess, system resources are for the moment devoted to Adobe from the first laggy document restore.

MS Excel? If I havent clicked on it for a while and then click on it, there will be 15-20 secs of lag before the spreadsheet pops up on screen. Clicking to save an image? Big lag calling up the Save As window. In general, the lag is most pronounced when switching from one app to another. But, look at what I have open. These are not processing-intensive apps. They shouldn't consume all computer resources when used.

This happens every single time, with every app. What could be causing it? Thanks in advance for any and all suggestions!