Among some of the problems you may encounter after upgrading from Windows XP to Vista, is slow keyboard input response. This tip from Windows Insight author describes the problem with a "setting in the i8042 driver—the driver which controls the PS/2 port—which isn't correctly migrated during the setup process" and causes sluggish keyboard response with a 100% CPU spike until it "catches up from the burst of typing".
The fix is simple but involves the following registry fix and a reboot…
Open the Registry and navigate to:
Right click on Parameters and select Export to backup the Key. Save it to a location on your PC that you can find if needed.
Select Parameters again, and in the right windows pane, look for the name PollStatusIterations. If it does not exist, create it using a new DWORD Value:
Set the value for the DWORD to 1, close REGEDIT and reboot.
This problem only occurs with PS/2 keyboards (that uses the I8042 port device driver). Also effected are USB and wireless keyboards plugged into the PS/2 port (using an adapter).