The Synaptics driver does evil, evil things to keyboards on some laptops. (It makes my Asus laptop keyboard practically unusable for a touch typist [drops 25% or more of keystrokes].) But, as soon as I uninstalled it, the keyboard went back to being fully responsive, and it never completely ignored all input, just got very sluggish. So, if you removed Synaptics already, I don't think that is your problem. Though, just to make sure, you might want to remove both the Synaptics device and your keyboard from device manager and reboot.
I don't know of SP3 affecting keyboards, at all, so I don't think that should be it. Rolling back SP3 would likely cause more problems than it's worth to attempt. So I'd say leave that alone.
And, as Trev said, clearing the ACPI isn't a bad idea when something isn't auto-detecting properly, as well.
