I would recommend to first check the wireless adapter and router settings (802.11 standard, channel, encryption…). Please make sure the encryption set in both devices is the same; preferably AES instead of TKIP.
You may also consider updating the router’s firmware and wireless adapter driver.
There are other variables that can affect:
Is the location of the router and laptop the same?
Is there any new electrical device or wall in the way which can create interference?