Hardware vendors are making updating drivers and related software a much more streamlined process these days, but certain installer packages such as ones by NVIDIA or AMD tend to leave file remnants behind after they finish running.
If have a GeForce card then you can look under "C:\NVIDIA" and "C:\Program Files\NVIDIA Corporation\Installer2". The first directory has older revisions of the installer that are kept behind and each can take about ~200MB of space - these are entirely safe to delete. While the more hidden path contains installer files for each driver component that's installed and depending on how frequent you update your drivers, the files can amass to several gigabytes (GB).
For Radeon video cards, go under "C:\ATI" instead. A single update can contain about 250MB worth of files.
Clearing these files can be particularly helpful if you are on a SSD and/or strapped for space on your main drive.