Press Ctrl-Alt-Delete to enter the Windows Task Manager. Click the 'Processes' tab. You should find a high figure (perhaps 80+ or 90+) against 'System Idle Process' , with everything else showing either '00' or a very low figure. If there's a program (other than System Idle Process) showing a very high figure it means that it's hogging all of the PC's processing power. (What you need to do about it will depend upon what it is. Post again if you're unsure).
Also go to Start>Run and type 'msconfig' (without the quote marks) into the box. Click 'OK' and then on the 'Startup' tab. There you'll see a list of every program which runs automatically whenever the PC is booted. Click to remove the ticks alongside anything not required. Click 'Apply' and 'OK'. Restart the computer. When you see a box stating that the computer is using 'Selective Startup Mode', click to put a tick in 'Do not show this message again'.
Chris