Paragon Rescue Kit 11Sure Free Edition is a professional disaster recovery toolkit that could be all you need to get an unbootable PC working again.
The software comes in the form of an ISO file, which the setup program will burn to CD or DVD. Boot your PC from this instead and you'll launch a collection of essential troubleshooting tools.
The Boot Corrector, for instance, is able to detect and fix problems with the Master Boot Record, partition table and Windows boot configuration data (either boot.ini or Boot Configuration Data files). It can even reach into the Registry of your broken PC, correcting drive letters and resolving other issues that may prevent Windows from loading.
You also get an Undelete Partition tool, which as you'll guess from the name, scans your PC for accidentally deleted partitions and recovers them for you.
And if these options don't work then, as a last resort, you can always turn to the File Transfer Wizard. This allows you to export important files and folders from your dead PC to another local drive, or burn them to CD or DVD, so at least you'll be able to restore your data on another PC.
This version of the Rescue Kit is free for non-commercial use. If you need more power, though, it might be worth looking at the Pro edition. This includes a Registry editor that allows you to manually look for and fix Registry issues on the broken system. The Network Configurator allows you to work with shared resources on the internet. If the boot problem is that you've forgotten your password then the Pro edition can remove it, and if all else fails a secure deletion module will wipe your hard drive so that nothing can be recovered. It's yours for $99.95, and you can find out more on the Paragon site.