So you were given a laptop, and to fix it, you installed Windows (I am assuming, since you said "wiped clean") and installed the drivers. You said you can connect to your network on WiFi, but you get no internal access? If that is the case, then the problem lies within your router/internet setup. If you can connect to an access point successfully, you installed the drivers correctly.
But just in case, download this driver, which is for your Inspiron Laptop, 6400
This is the Network Driver for Dell Wireless 1370/1470/1390/1490/1505/1395 WLAN Driver (I'm assuming you want to download this driver)
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This is the Network Driver for Intel Intel(R) PRO/Wireless 3945ABG Network Connection, (R) Wireless WiFi Link 4965AGN, v.11.5.0.0 (TIC148234)
http/