Trouble connecting via airport at home and at work

Until very recently I had no issues at all connecting via Airport both at work and at home. About a month ago the connection just stopped, I had not installed anything new. At the time I was running Snow Leopard 10.6.2. After trying various fixes I noticed that the 10.6.3 update came out and said i fixed various Airport issues. So I carried on and updated. Still no joy. Basically Airport polls the wireless signals constantly but does not find any. Computers beside me at work can see a whole multitude of wireless signals. I have created a new Airport connection, a new location, I have zapped the PRAM and SMC. I have deleted various preferences in System Configuration but no joy. I have thought from day one that this might be hardware related but I have found that difficult to prove. I was at home the other night and set up the laptop literally within 500mm of the router and it found the wireless signal no problem. It stayed up as long as I did not move the laptop any further away. When I did it lost the signal and asked for the password. Inputting password just gets a timeout error. I tried the same at work yesterday and the same thing happens, when I am close to the router I get a wireless connection but move it no more than 2-3 metres away and I lose the signal. Surely this means it is hardware. The MacBook Pro is nearly 4 years old so I have no warranty. Can the internal airport card be replaced? If so how and where do I get one? Any help would be greatly appreciated.