How to restict users from entering special characters in Microsoft Dynamics CRM 4.0

Solution is very simple - just add following script to OnLoad event handler of form:

function SwitchOnCheck(ElementId)
{
    var element = document.getElementById(ElementId);
    if (element != null)
        element.attachEvent("onkeyup", function()
{
    var mikExp = /[$\\@\\\#%\^\&\*\(\)\[\]\+\_\{\}\`\~\=\|]/;
    var strPass = element.value;
    if (strPass == null)
        return;
    var strLength = strPass.length;
    var lchar = element.value.charAt(strLength - 1);
    while(lchar.search(mikExp) != -1) 
    {
        strPass = strPass.substring(0, strLength - 1);
        if (strPass.length == 0)
            break;
        strLength = strPass.length;
        lchar = strPass.charAt(strLength - 1);
       }

    element.value = strPass;
});        
}

SwitchOnCheck('firstname');
SwitchOnCheck('lastname');