System Hangs when a program is installed

I have a system that hangs everytime a program is installed that needs to reboot. If it doesn’t it works fine.
I am running ME
Duron 1.3GHZ
ECS K75SA Mainboard
128MB PC133
20GB 5400RPM

I have changed mainboard, ram, CPU, system has 2 case fans and is running cool.

Any ideas??