First you need a tool called "Resource Hacker". This free program allows you to change resources in any .exe file such as "Explorer.exe", which includes the [Start] button's Label. You can visit Download.com and search there for "Resource Hacker" (Click Here to Download) After you download it, follow the guide here:--
Step 1: =====
A - Run "Resource Hacker" and open the file "%windir%\Explorer.exe". B - You see a Tree of all Resources in this file, expand the "String Table" C - Find the "start" and replace it with your own text. then press the [Compile Script] button. D - Save "Explorer.exe" as "MyStart.exe" DONT save it as Explorer.exe, do "save as.." and give it a new name. E - Quit "Resource Hacker".
Step 2: =====
A - Click on the [Start] button and choose the "Run..." item from the start menu. (Or use the shortcut key WinKey+R) B - Type "RegEdit" in the Run "Dialog Box". And then press the [Ok] buton to run the "Registry Editor" program. C - Go to: "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\WindowsNT\CurrentVersion\Winlogon" and find the "Shell" property. D - Replace value of the "Shell" property to "MyStart.exe". E - Quit "Registry Editor". F - Restart your system. Note about Registry Editor: if you did not find the key "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\WindowsNT\CurrentVersion\Winlogon", you can search the Registry for the "Explorer.exe", to do this use the Edit Menu | Find Next (Ctrl+F).