Unexpected shutdowns and unresponsive browsers

I have a Dell computer and am using Windows 7. About a month ago my computer started unexpectedly shutting down. This never occurs when I am actively doing something online. But when I stop, I will often get an "unexpected shutdown." Now I cannot even get online using Opera, Firefox, or Chrome. When I try, I get nothing (with Firefox) or a message of "Not Responding." I did a scan with Sophos and found no problems. But I cannot even get Malwarebytes to run a scan. It goes for three or four seconds and then stops. When I hit "Abort Scan," I get a "Not Responding" message. I can access the net through Explorer, but not always.

Thanks for any suggestions...



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) i7-2600 CPU @ 3.40GHz, Intel64 Family 6 Model 42 Stepping 7
Processor Count: 8
RAM: 8174 Mb
Graphics Card: AMD RADEON HD 6450, 1024 Mb
Hard Drives: C: Total - 939783 MB, Free - 832026 MB;
Motherboard: Dell Inc., 0Y2MRG
Antivirus: Sophos Anti-Virus, Updated and Enabled