I just watched a Dell support tech fix my computer that had the same problem on Vista Home Primium SP1. His solution was to uninstall the device driver from Device Manager, then reboot to let Windows re-install the driver.
Open Device Manager, expand IDE ATA/ATAPI controllers, then right click each entry for ATA Channel 1 and click Uninstall. Reboot.
When the computer restarts it will reinstall the drivers. In my case, it solved the problem and my computer now enters Hibernation (Sleep) mode.