Boot Repair Disk is an open source rescue disk which can fix many common Windows and Linux boot problems (boot sector, master boot record, GRUB, more).
While this can be a horribly complex area, Boot Repair keeps everything very simple. We chose to run a 64-bit session, selected a language, and waited for a while as our system was checked. A dialog then appeared with a "Recommended repair" button, and clicking this should fix the most common problems.
If you need full control - or, understandably, you don't trust this kind of program and want to know what it's doing - clicking Advanced Options tells you more. We found tools to back up our partition table and boot sector, restore the MBR or boot menu, repair our file system and fix Windows boot files.
If that still doesn't help, a "Bootinfo Summary" option creates a detailed report on your system drive. Conveniently, this is posted online at a custom paste.ubuntu.com URL so you can immediately share it with others (this is safe as there's no personal or identifying information, but if you're worried you can turn it off).
The disk is also a LiveCD, so if your system has access to the internet then you'll be able to browse the web for further assistance. Experts will find other useful tools available - GParted, OS-Uninstaller - which may help them to resolve the problem manually.
An excellent rescue disk, easy to use for beginners yet with real power and depth for the more experienced.