Kindly help with antiviruses….

hi Friends..we have a network of some 30 systems running windows xp and one windows server 2003.we have all variety of hardwares like 256 Mb ram,512 Mb ram and 1 Gb ram too.I am often getting problems from my clients like cannot connect to server and many problems that cause the system to malfunction.tried few antivirus but nothing helps me.Kindly suggest me a good antivirus and effective one which can be applied to my network and which will be free of cost and can be of frequent update type.waiting for your kind replies.