I recently installed Windows 7 (64-bit) using retail media on my T400. It previously had Windows XP (32 bit)and I had to do a clean install. That means I lost all the Thinkvantage add-ons (aka bloatware) and it didn’t bother me too much since I like only a few of those utilities, which I could individually install.
After installing the OS and the drivers available from the T400 support site, the Wi-Fi device (Intel WiFi Link 5350) won’t start. It showed up with problems in the device manager. I checked the support site and there was only one driver available for this device. After trying various options (including restarts) unsuccessfully, I went to the Intel site looking for drivers. There were multiple versions available. I chose to install the latest driver and voila! The device was working in no time and was connected using 802.11n. I haven’t had any driver related wireless problems since.