Dell Studio 1555 laptop : Wireless Network Adapter issue "the capability of your network adapter does not match the requirements of this network"

I have an issue with my Intel Wifi link 5100 agn wireless network adapter. It does not connect to my wireless router and displays the following error "the capability of your network adapter does not match the requirements of this network"

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Dell Studio Laptop 15 configuration:-

OS : Microsoft Windows Vista 64-bit

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Intel Wifi link 5100 agn wireless network adapter Configuration:-

Current Driver version : 12.0.0.1 (Latest driver installed)

Driver signature : 2009

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DLink wireless N-Series router :-

Model : DIR-615

Supports : a,b,g,n IEEE 802.11 standards

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Problem is that it does not work at all if i configure my router with N-standard. But for G-standard it worked for sometime but again it kept on disconnecting.

I tried to update the wireless network adapter drivers but still it does not worked for N-standards.

What other things can i do in my router  inorder to maintain steady wireless connection ? Is it interefering the signalling with other routers around my home ?

I have removed all the hard core security settings like 'filtering by Mac Address', 'SSID invisiblity' ,etc.

Only one security is kept i.e. WPA-Personal encryption but still same problem is there.

I would really appreciate your help.

Thanks in advance