I must write this as a answer to my question, because another may get this issue. I solved this issue by following code. It is working fine.
// It is a good idea to define the variables at the beginning of each event.
// This allows you to include conditional statements using the crmForm.FormType property.
var CRM_FORM_TYPE_CREATE = 1;
var CRM_FORM_TYPE_UPDATE = 2;
var CRM_FORM_TYPE_READ_ONLY = 3;
var CRM_FORM_TYPE_DISABLED = 4;
var CRM_FORM_TYPE_QUICK_CREATE = 5;
var CRM_FORM_TYPE_BULK_EDIT = 6;
var oField=crmForm.all.new_projectstatus;
//Only create the pop-up message if the user can edit or update the form.
//Do not show the pop-up message in the Quick Create form.
switch (crmForm.FormType)
{
case CRM_FORM_TYPE_CREATE :
case CRM_FORM_TYPE_UPDATE :
case CRM_FORM_TYPE_BULK_EDIT :
//Create a single popup object on the form that can be re-used by
// different functions or events in the form.
var oPopup = window.createPopup();
var oPopupBody = oPopup.document.body;
var width = 250;
var height = 0;
var strInnerHtml=null;
oPopupBody.style.backgroundColor = '#FFFFE8';
oPopupBody.style.fontFamily = 'Arial';
oPopupBody.style.border = '1px solid black';
oPopupBody.style.fontSize='11px';
oPopupBody.style.color='#000000';
//Create the function that will be attached to the field mouseover event.
function Show_Tooltip ()
{
//Get the selected datavalue
var selectedFieldValue=oField.DataValue;
// Set the message HTML as per the selected value.
if(selectedFieldValue==1)
{
strInnerHtml="Have made contact with someone from the company regarding";
strInnerHtml= strInnerHtml+"this scope of work. value or dates may not be available.";
height = 50;
}
else
if(selectedFieldValue==2)
{
strInnerHtml= "FO have put forward a proposal (proactive or reactive)to the";
strInnerHtml= strInnerHtml+"client. Waiting on response";
height = 35;
}
else
if(selectedFieldValue==3)
{
strInnerHtml = "Official expression of interest for a clearly defined scope of work has been";
strInnerHtml= strInnerHtml+"sent to the client";
height = 38;
}
else
if(selectedFieldValue==4)
{
strInnerHtml = "Post proposal/ EOI Discussions with customer. Dates & Value should be";
strInnerHtml= strInnerHtml+"known & added to the CRM.";
height = 48;
}
else
if(selectedFieldValue==5)
{
strInnerHtml = "Project has been put on hold by the customer. Estimated date of re-";
strInnerHtml= strInnerHtml+"instatement or reminder for follow–up should be put into the";
strInnerHtml= strInnerHtml+"CRM";
height = 50;
}
else
if(selectedFieldValue==6)
{
strInnerHtml = "FO has been requested to submit an official Quote/Tender for the scope of";
strInnerHtml= strInnerHtml+"work. Please convert this OPPORTUNITY to QUOTE/TENDER";
height = 52;
}
// Set the location and size of the pop-up message.
// The x and y coordinates are relative to the form element.
var x_coord = -80;
var y_coord = 20;
// Associate the pop-up message with the element where the event occurred.
var documentElement = event.srcElement;
// Show the pop-up message.
oPopupBody.innerHTML = strInnerHtml;
oPopup.show(x_coord, y_coord, width, height, documentElement);
}
// Attach the onmouseover,onkeydown,onkeyup events to the field and the function.
// It is important that this be done in script after the function is defined.
crmForm.all.new_projectstatus.attachEvent('onmouseover', Show_Tooltip);
crmForm.all.new_projectstatus.attachEvent('onkeydown', Show_Tooltip);
crmForm.all.new_projectstatus.attachEvent('onkeyup', Show_Tooltip);
}