[Callum]

To turn on FilterKeys
FilterKeys is a keyboard feature that instructs the keyboard to ignore brief or repeated keystrokes. Using FilterKeys, you can also slow the rate at which a key repeats when you hold it down.

Open Accessibility Options.
On the Keyboard tab, under FilterKeys, select the Use FilterKeys check box.
To turn off FilterKeys, clear the Use FilterKeys check box.

Notes

To open Accessibility Options, click Start, click Control Panel, and then click Accessibility Options.
If the Use shortcut check box in the Settings for FilterKeys dialog box is selected, you can turn FilterKeys on or off by holding down the right SHIFT key for 8 to 16 seconds (depending on the other settings you have selected in the Accessibility Options dialog box). To open the Settings for FilterKeys dialog box, open Accessibility Options and then, under FilterKeys, click Settings.

Presumably... you can turn it off the same way...