Updating your drivers regularly is par for the course of running a well-maintained PC. Most installers are very good at cleaning up after themselves, though there are some notable exceptions. NVIDIA's "Forceware" driver package, while painless to use, leaves large files just sitting on your hard drive that can be safely deleted.
There are actually two directories you can confidently nuke once an NVIDIA driver install is complete. The first one you probably already know:
This is the default extraction path for NVIDIA's installers and if you're a regular updater, you might find a number of folders holding old driver revisions, each taking up 100-200MB of space. These are safe to delete, though you can probably keep the most recent one or two, just in case you need to do a roll-back.
The second directory is a little hidden away, and you'll need access to the "Program Files" folder of your Windows drive. The path you want to navigate to is this one:
C:\Program Files\NVIDIA Corporation\Installer2
Now this is the more problematic of the two -- at least with the first path, you're made aware of it by the installer. One the other hand, this folder can accumulate files for months, even years and you'd have no idea it's happening without the help of a space visualiser.
When I checked this directory out, it contained 2.9GB of old installer files. That's a fat chunk of space in anyone's books. The good news is you can just delete the entire contents of this folder and no harm will come to your system. All you'll get in return is a healthy portion of your hard drive back.