I came across the same problem on a FW590C. I reinstalled the system with Windows 7 ultimate and couldn't get the keys to respond. I placed a call to Sony tech support and was given the following instructions and was able to resolve the issue:
1. Go to the sony vaio drivers website and select the model of laptop and OS you require. I am not sure if the same drivers that apply to my laptop are available for all models, but you can check.
2. Download following drivers and install them in the order in which they are listed:
- Original Sony Shared Library (once installed reboot the system)
- Original Sony Notebook Utilities (once installed reboot the system)
- VAIO Content Components (no reboot require after this driver)
- Setting Utilities Series Update (once installed reboot the system)
- VAIO event service update (once installed reboot the system)
3. Upon system restart (following the install of the VAIO event service update) go to services and ensure that the VAIO event service is set to automatic and started.
It took a while to complete the install due to all the reboots, but in the end all the keys including the S1, AV/Mode, and FN keys are working again.