If it indeed is the motherboard, finding a replacement LGA 775 board that you can use with your existing memory, CPU/HSF, etc. might be a bit of a challenge. Especially if you are using the x16, x16 SLI your old board provides.
Your board ( http://usa.asus.com/Motherboards/Intel_Socket_775/P5K/#specifications
) may only be available as a used item somewhere.
Here are some LGA 775 boards available at EBuyer if you're in the UK: http://www.ebuyer.com/search?page=1&store=2&cat=14&filtersubcat=658
And some LGA 775 boards available at Newegg if you are in the US: http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=100007627%20600007879&IsNodeId=1&bop=And&Order=RATING&PageSize=20
But be sure to verify they use DDR2 memory and not DDR3.
You may have better luck looking for a good used board someplace like eBay. I've always liked Asus, Gigabyte, and Asrock boards, myself.