You Can't Access the Hard Drive
If Windows can't boot because the PC can't read the hard drive, none of the solutions above will work. But that's not the worst of it: Unless you have a very up-to-date backup, all of your files are locked away on a possibly dead hard drive. Secondary drives you don't boot off of, both internal and external, also can die with important data locked away on them.

If you can't access your hard drive, Recover My Files might be able to do what its name implies.

If the drive is making noises that you've never heard before, shut off the PC immediately. In that case you have only one possible solution, and it's expensive: Send the drive to a data-retrieval service. Drivesavers and Kroll Ontrack are the best known, although they're not necessarily better than smaller, cheaper companies. Expect to pay hundreds or even thousands of dollars. If your drive sounds okay, however, you may be able to recover the files for only $70 with GetData's Recover My Files.
If the sick drive is the one you use to boot Windows, you'll have to remove it from the PC and access it on another computer. You can do so by making it a secondary drive in a desktop PC, or by using a SATA-USB adapter such as the Bytecc USB 2.0 to IDE/SATA Adapter Kit.
The free, demo version of Recover My Files will show you which files can be recovered (almost all of them, when I tested it) and even display their contents. Once you've paid the $70 license fee, the program can copy the files to another drive. If that doesn't work, you'll need to use a retrieval service.