PC Freezes

About 3 months ago my Dell tower, running Windows 7 Home Pro with dual monitors and cordless mouse, started freezing when using the Internet. It is at the point where it freezes almost instantly.

It doesn't matter what browser I used: IE, Chrome or Firefox, they all freeze.
No applications freeze - Quicken, Word or Excel.

I had some issues with my home network router so I replaced it with a new DLink router.
It still froze, usually as soon as you opened it up. Sometimes you use it for a sort while before it froze.
I ran virus programs and spyware detectors which didn't find anything.
I had the peripherals plugged into a KVM unit to control a second PC, so I detached everything and hooked them directly into the tower.
It still froze.
After discussing it with a PC guru, he suggested we wipe the tower clean and start from scratch.
It took me almost a full day to get the software I needed back on - the key pieces are Quicken and Quickbooks, my wife does some bookkeeping for a local company.
It still froze when going on the Internet.
I removed the dual display card and plugged the main monitor into the native display adapter in the tower.
It still froze.
I had been using a Logitect wireless mouse so I removed that and put a regular cord powered mouse it its place.
No change.
As a last ditch effort I changed the USB port the mouse was using but that didn't fix it.

I don't see anything left to change or replace.
Any help would be appreciated.

Here are the system details:
Tech Support Guy System Info Utility version 1.0.0.2
OS Version: Microsoft Windows 7 Professional, Service Pack 1, 64 bit
Processor: Intel(R) Core(TM) i3-3220 CPU @ 3.30GHz, Intel64 Family 6 Model 58 Stepping 9
Processor Count: 4
RAM: 3967 Mb
Graphics Card: Intel(R) HD Graphics, 1791 Mb
Hard Drives: C: Total - 476709 MB, Free - 411506 MB;
Motherboard: Dell Inc., 0XR1GT
Antivirus: avast! Antivirus, Updated and Enabled

 

i would test the memory........... u might have a bad stick
download memtest
make a bootable cd or thumbdrive
then let run for a few hours