I held a key down while I was talking to someone on the phone. Now the key response is very slow and makes a clicking sound as the letter appears??

This is an educated guess on my part, but perhaps you invoked one of the "Accessibility Options" in Windows. (I'm assuming we're talking about Windows here).

Go to Control Panel->Accessibility Options and see if any of the features are enabled. Then just turn them off and see if that helps.

Good luck,
WallyH