On a computer with internet access, go to support.lenovo.com, click Laptops, select "ThinkPad T Series laptops" and select "ThinkPad T420" subseries/model, click Go. Select the relevant operating system (Windows 7 64-bit or 32-bit) in the pull-down menu. Proceed to download all the drivers that you want. You can download these files on a different computer than the one that you will install the drivers on, and then transfer them via external harddisk, or similar, to the computer that you want to install them on.
The folder C:\SWTOOLS (and other, for example C:\DRIVERS) will be created by the self-extracting archives that you download from support.lenovo.com (the .exe-files), if you choose to extract them to their default location, that is. These folders then contain the installation files for the drivers. After the drivers are installed, both the archive (.exe-file) and the installation files may be removed, but then you would have to download the archive anew should you ever want to install the driver again. But that may be a good thing, since the driver may have been updated since the last time. Usually the drivers are not updated by Windows Update (some are, however). The ThinkVantage System Update utility updates the drivers, at least semi-automatically. But these (Windows Update and ThinkVantage System Update) both require an internet connection, of course.
The most important drivers are the chipset drivers ("Intel Chipset Support" under "Chipset" component), SATA drivers ("Intel Rapid Storage Technology Driver" under "Storage" component), graphics driver (Intel HD Graphics or NVIDIA Optimus Display driver (depending on which one you have), and perhaps the monitor INF file for better performance), and various hardware components. Bloatware and enterprise utils (for example remote management utils) are not necessary to install, at least if you don't have explicit need for them.
These drivers do not require an internet connection to install, but you need an internet connection to download them, naturally. Read the README for installation instructions, because not all of the drivers are "click-and-play". Some of them you have to install manually (even more manually than usual) via the Device Manager (e.g. probably the Intel SATA driver) or otherwise (the monitor INF file).
You should, however, also update Windows via the Windows Update service. For that, you should use an active internet connection. There are also offline windows update possibilities, for example WSUS Offline Update, but I would recommend the standard online update (move the computer temporarily to an internet access point, for example).
Another problem is that it seems that a few Lenovo's drivers (e.g. the Bluetooth driver and the Ricoh Multi Card Reader driver) are unsigned, and Windows (at least an updated Windows) will block the drivers (and probably the device as well). This I experienced on my ThinkPad T61p, which I installed yesterday (12 October 2014).