Windows 2000 server freezes

Hi
I have 5 clone computer running windows 2000 ADV. server in my home Network; built with AMD and Intel processors. each of them has 256 MB memory and 8GB HD, only 2 GB used, NTFS partition.
My problem is my computers freezes frequently. any application I run it takes long time, if I try to run more than one program It freezes. Task manager does not help, only way is either logof or restart. it is brand new installation so Virus is not an issue. Please help me.
thanks
MD