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How to install ahci support for ati sb600 sata controllers on existing windows installations
Back up your critical data, and especially your registry.
There are lots of programs out there to do it. Hopefully you won't have to use your backup, and the procedure is pretty safe, but consider yourself warned anyway.
Make sure ahci is disabled in the bios. If it's enabled and you can boot into windows, you don't need this walkthrough.