Assuming that it's a desktop model (rather than a laptop), simply borrowing another keyboard will allow you to find out if it's a keyboard fault. (If it is you can buy a replacement keyboard for not much more than a fiver in many big supermarkets).
If it's not a keyboard fault there could be a program running on your computer which is hogging all of the processing power. Press Ctrl, Alt and Delete together to get into the Task Manager (or go to Start>Run, type 'taskmgr' into the box and click 'OK'). Click on the 'Processes' tab and look down the 'CPU' column. All of the entries (except 'System Idle Process') should have very low numbers next to them. ('System Idle Process' should have a figure in the 90s). If anything unusual has a high figure, click on it, then on 'End Process' and accept the warning message. (If it keeps coming back, post again to tell us what it is).
Chris