If you can try get a desk lamp next to it to add some extra light, also pump the ISO too 800, it'l be quite grainy but I find the D40's grain to be kind of 'filmesque' so it doesn't look to bad.... maybe even try 1600 if you can't get a fast enough shutter speed.

Also, if you're pointing at the cat and taking a light metre reading you're fighting a losing battle... point it at something white, or generally a bright colour, and take a reading from that,  set the right shutter speed, try an aperture of about 4-5.6, that should let enough light in for a fast shutter speed at that ISO

If you need anymore info just ask, or send me a message if you like

Chris if he takes his reading from white his cat will not have any detail in it at all!

Try not using the flash, take your light reading from the cat (using spot meter if you can). If your cat is black, your camera will try and make that tone mid-grey, so set the shutter speed around 2 and a half stops faster, and this should place your cat at black with detail.

My best advice is though, your using a digital camera, just experiment. Taking 500 pictures as oppose to taking 10 doesn't cost you any different.