If all you want is to change the title of the Windows 7 menu to look like XP, I don't think that's possible, even with a custom theme. The layout is just too different.
There are several tools that let you get the classic start menu on Windows 7. I recommend my own creation Classic Shell:
http://classicshell.sourceforge.net/index.html
Classic Shell has a skin that makes it similar to the XP start menu, but there are some key differences (like no recent programs, no pinned programs, and no shutdown buttons at the bottom). Check out the XP Luna skin here:
Classic Shell: Gallery
But since one of your requirements is "without losing functionality", you probably don't want to lose the search. I'm aware of only one tool that has search:
Windows 7 Classic Start Menu. It is not free though.