I had this problem and found changing the wireless channel on the router resolved it. I don't know what caused the problem as there are no other wireless networks using the same channel but it worked.
What did you disable on task bar...the Windows wireless icon or do you have the Ralink Wireless Utility?
Can I ask what version of Windows you are running? Although this isn't likely to be the cause of your problem, I found recently that my RT2500 wireless card was only connecting at 11 Meg instead of 54 Meg and rquird a driver upgrade for Vista.