The Windows XP boot menu is determined by the contents of C:\boot.ini, a scary little text file that nevertheless is pretty important to know about if you fix computers. Microsoft changed how the boot menu works in Windows Vista and 7, helpfully removing the Edit button as you can see in the screenshots above.
You’re meant to use the even scarier command-line-only tool bcdedit, but I can’t be bothered learning how that works just to fix the incredibly basic problems I usually see (duplicate entries due to messed-up OS installs or repairs, or tweaking dualboot systems properly).
I’d like to share a lazier solution: VistaBootPRO gives you a perfectly good GUI for boot menu editing. The personal edition sells for US$10 on their site, but there appears to be a free version available here if you’re only going to need it once or twice.
It gives me a slightly scary error message on startup about Vista not being detected (I’m running Windows 7) and that I may “experience minor problems”, but it’s worked fine for the basic things I’ve done with it. YMMV.