Have a look in your router's setup options and look for an option to change the channel (usually numbered from 1 through to 10/11).
Make a note of what the default setting is and then work out which would be the furthest channel from the default that you could change it to.
For example. if the default is 7 and you have channels 1-10, the furthest would be channel 1.
Change to that channel, power off the router and the computer.
Power the router up and wait for the lights to settle, now power up the computer, let the HDD activity light settle and then log in.
Establish the wireless connection and go about your normal business
With a bit of luck this should sort things out, not sure how but it worked for me. My router's a Linksys one, I had nothing but trouble for the first day or so - then I changed the channel and everything's been fine for the last 9 months
__________________
Might also want to check that you haven't got any cordless phones or other sources of interference about as well
