How should I identify bradcast in my network due to virus I am having 100 PC in my network I am using os windows xp,windows98,windows 2000 guide me for free tools

Use Wireshark. It is a free Network Protocol Analyzer.

http://www.wireshark.org

I want to create welcome logon script
When user logon it should give time & welcome department

Here it is for a VBScript logon script. It uses the Voice API to speak to you or you can change the script to use a message popup instead. I reads the Active Directory user information and gets the Given Name only to create a personalized greeting. For the full name, you would have to add something like:
strFullName = CurrentUser.GivenName & " " & CurrentUser.sn

Here is the script below:

Set ADSysInfo = CreateObject("ADSystemInfo")
Set CurrentUser = GetObject("LDAP://" & ADSysInfo.UserName)
strGivenName = CurrentUser.givenName
strDepartment = CurrentUser.Department

VoiceGreeting

Sub VoiceGreeting

Dim HourNow, Greeting
HourNow = Hour(Now)
If HourNow >5 And HourNow <12 Then
Greeting = "Good Morning "
Elseif HourNow >12 And HourNow <16 Then
Greeting = "Good Afternoon "
Else
Greeting = "Good Evening "
End If

Dim oVo
Set oVo = Wscript.CreateObject("SAPI.SpVoice")
if strDepartment = "" then
oVo.speak Greeting & strGivenName & ". Tell the Help Desk to fill in the department name in your user account and get to work."
else
oVo.speak Greeting & strGivenName & " of the " & strDepartment & " department. Now get to work."
end if

End Sub