grepWin allows you to perform powerful search and replace functions with a simple, intuitive user interface.
It will add a context menu to the Windows Explorer shell to easily search selected folders. Once started, grepWin allows you to customize the search or replace in many ways: File sizes, file types, Case Sensitive, Include/exclude system folders, etc...
It also has a 'create backup' function, which can be VERY handy with bulk renaming.
Since regular expressions can sometimes get complicated, grepWin allows you to add your most used expressions to a presets list.