On a native Windows install, you can either use batch(cmd.exe) or vbscript without the need to get external tools. Here's an example in vbscript:
Set objFS = CreateObject("Scripting.FileSystemObject")
strFile = "c:\test\file.txt"
Set objFile = objFS.OpenTextFile(strFile)
Do Until objFile.AtEndOfStream
strLine = objFile.ReadLine
If InStr(strLine,"ex3")> 0 Then
strLine = Replace(strLine,"ex3","ex5")
End If
WScript.Echo strLine
Loop
Save as myreplace.vbs and on the command line:
c:\test> cscript /nologo myreplace.vbs > newfile
c:\test> ren newfile file.txt