System is very very slow

What is problem of the system is hang after 1 and 1/2 hour

There are many causes to that simptom (assuming you are using any version of windows):
1 - a disk full of temporary internet files, and loads of other junk used once by the operating system;
2 - a disk that has not been defragmented for a long time
3 - dust! it gets in all the crevices of the machine, and it overheats, making it slower and slower. Recently I vacuumed a computer at work and brought it from 85 centigrade degrees to 41 degrees, just by removing dust in the cpu cooler, and it brought a major improvement in performance.
And some other causes, but these are the most typical.