var fieldToFocus;
function computeForm(form) {
  error = false;

if (error == false && is_inputValid(form.UserName,"Enter User Name",2,"EmailChar"))
  { 
      error = true;                                       
  }
if (error == false && is_inputValid(form.UserPassword,"Enter Password",2,"LettersAndNumbers"))
  { 
      error = true;                                       
  }

  if (error)
     return false;
    else
     return true;
}


<!-- To check if the input is blank-->
	function isBlank(field,msg)
	{
		if (field.value == "") {
    			alert ("You Must Enter "+msg);
    			field.focus();
    			field.select();
    			return true ;
 		} 
		else 
		{
   	 		return false ;
 		}
	}
<!-- Validate input -->
	function is_inputValid(input_field,msg,cSize,dataType)
	{
		var validchars
		var letter
		if (dataType == "Letters")
			{
				validchars = "abcdefghijklmnopqrstuvwxyz";
			}
		else if (dataType == "LettersAndNumbers")
			{
				validchars = "abcdefghijklmnopqrstuvwxyz0123456789";
			}
		else if (dataType == "LettersAndNumbersWithSpace")
			{
				validchars = "abcdefghijklmnopqrstuvwxyz0123456789 ";
			}			
		else if (dataType == "Numbers")
			{
				validchars = "0123456789";
			}
		else if (dataType = "EmailChar")
			{
				validchars = "abcdefghijklmnopqrstuvwxyz0123456789@._-";
			}
		else
			{
				validchars = "abcdefghijklmnopqrstuvwxyz0123456789";
				
			}
				
		if (input_field.value.length == 0 )
			{
				alert("You Must " + msg);	
    			input_field.focus();
    			input_field.select();				
				return true;				
			}
		else if (input_field.value.length < cSize) 
			{
				alert("Minimum " + cSize + " character long, please try again." );
				input_field.focus();
				input_field.select();
				return true;
			}			
		else
			{
				for (var i=0; i < input_field.value.length; i++) 
					{
			    		letter = input_field.value.charAt(i).toLowerCase();															
					    if (validchars.indexOf(letter) < 0)
							{				
								if (letter == " ")
									{
										letter = "SPACE"
									}									
						    	alert("Contains Invalid Character: " + letter);			
								input_field.focus();
								input_field.select();								
						   		return true ;
							}
					}
				return false;
			}
	}