Have downloaded the software and the benchmark test causes a freeze when the dial in top right corner reaches 2/3s and task manager CPU Usage under performance reads 100%.
When benchmark process is over, CPU usage returns to about 5% and the it unfreezes.
What does this mean and how do I solve the problem?
That is normal behaviour for sandra. Run a stress test with prime95 on 'in-place large fft' setting. If that passes, next you could make a bootable cd with memtest and check the memory.