Constant Freezing High Disk Usage

I run a system I built back in 2009 with the following specs:

Tech Support Guy System Info Utility version 1.0.0.2
OS Version: Microsoft Windows 7 Home Premium, Service Pack 1, 64 bit
Processor: Intel(R) Core(TM)2 Quad CPU Q9650 @ 3.00GHz, Intel64 Family 6 Model 23 Stepping 10
Processor Count: 4
RAM: 3069 Mb
Graphics Card: NVIDIA GeForce GTX 470, 1280 Mb
Hard Drives: C: Total - 610476 MB, Free - 178886 MB; G: Total - 476936 MB, Free - 138294 MB;
Motherboard: ASUSTeK Computer INC., P5Q DELUXE
Antivirus: Norton Security Suite, Updated and Enabled

FYI, I have 4 GB installed, but I've never been able to get that last GB of RAM to be used (and I have made sure that my BIOS is set up to use it).

As time has gone on my computer has more and more moments when it just locks up; and it is also taking longer and longer to log in/switch users and for programs to open. My memory usage generally is below 70%, but I regularly get high disk usage warnings from Norton. Also the computer will freeze up when Firefox goes to certain sites like Comcast.net.

I've been trying to figure this out for a very long time and have gotten nowhere. I really appreciate your help.

Daniel