Wireless connection problems Between Dlink router and Windows Vista

I have a client who has just bought a Dell P.C with windows Vista home premium.  He also bought a DLINK DSL 2640B wireless router which came with a wireless USB adaptor.  I set the router up for him and it all connected fine.  I used WPA-PSK to secure the wireless connection and for a week it was all fine, then my client stated that the internet connection got gradaually slower and slower and then just died.  So I went back out armed with all the settings from his internet provider to make sure router had the right Primary and secondary DNS servers configured set it all up and it connected quick as you like.  That was Wednesday.  Now I've had a phone call saying that its slowing down again and that his wife has been trying to get on the tesco site all morning to no avail.  He told me that when he got in the network I con Vista places in the sys tray had a cross through it which suggests to me that the wireless adaptor isnt connecting for some reason.  I have run all the checks and everything is set up fine does any one have any suggestions as to what this could be.  I have run extensive virus and spyware scans and nothing comes up.  And when I was there last I check the IPconifg and everything was as it should be. Helllllllllllllllp