Dell laptop freezes

I have an aging Dell Studio 1735 with Vista Home Premium that now has a Samsung SSD hard drive showing 66gb free of the 111gb available. Recently it has taken to freezing. Leaving it alone for two hours did not solve the problem and it has to be hard booted. The PC is not on 24/7 more like 4/7 and has the free Avast Antivirus and I have run Malwarebytes Anti-Malware which only found lots of PUPs. I ran Windows Defender which found nothing. I downloaded the free Process Lasso and currently with only that running plus notepad it says there are 788 threads in 68 processes. the CPU utilisation is between 10-21%, responsiveness is 100%, RAM load is at 59% of 2GB usable RAM but I am not sure if all of that is ok. It has had four restraints today from iexplore.exe and jp2launcher.exe. Top of the restraints are due to iexplore.exe (299) and firefox.exe (86). Someone suggested that WMIprvse was a problem with high CPU usage and restarting the Windows Management Instrumentation does not seem to have any affect on the CPU. I have had the back off and air sprayed it. I downloaded CPUID HWMonitor but nothing is over 41?C. Backed up registry and ran Ccleaner. Firefox does seem to be the culprit but it once froze when I was closing the computer down for the night. Even so I uninstalled and reinstalled it but no change. Some web pages take an age to load.
Ideas anyone.

Dell laptop freezes