Thanks to everyone who has replied. In answer:
islandmon - I could not see anything untoward in the 'Activity Monitor'.
brudgers - the keyboard is working fine, and i'm pressing down on the whole part of the key.
Ray and rDesign - I have tried the suggestions, and do not use (nor have) Quicken.
Out of interest, I also experience problems with keys in Photoshop, so maybe there is something else running in the background that's preventing the use of the keys. Any suggestions would be appreciated.
Thanks,
Matt