If your PC won’t boot properly then running Startup Repair from the Windows DVD will fix many common problems.
But if you can’t find your disc, or you never had one, or you run Startup Repair and it just doesn’t work, then you should take a look at Boot Repair Disk.
Boot Repair Disk is an open source rescue disk which can detect and fix many Windows and Linux boot problems: a corrupt boot sector, master boot record, GRUB issues, whatever (almost) they might be.
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 at a minimum we could just click this and wait for the fixes to be applied.
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. (There’s no personal or identifying information, but if you don’t like this feature anyway then it can be disabled.)
Boot Repair 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.
Put it all together and Boot Repair Disk is an excellent package, supremely easy for novices to use yet with real power and depth for the more experienced.