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Device drivers for Windows come in two basic varieties: signed and unsigned. Signed drivers have been verified by Microsoft as stable and valid through its Windows Hardware Quality Labs (WHQL) program. The reason they're called "signed" drivers is because they have been labeled with a digital signature, which authorizes Windows to make the driver and its support files part of the operating system.
Unsigned drivers are offered directly by a hardware manufacturer. While they usually work fine, Microsoft has not tested them for full device compatibility or stability. They have no cryptographic signature, hence the name. Administrators can install unsigned drivers (although the system will issue a warning when you do), but ordinary users cannot.
Sometimes you can choose between installing one type of driver or the other for a particular piece of hardware. The choices often break down not two but three ways: a WHQL-certified driver from the hardware maker, a WHQL-certified driver from Microsoft/Windows Update and an unsigned driver from the hardware maker. Video adapters are a very common type of hardware for which you might see all three choices, so I'll use those for my example.
The Microsoft-issued WHQL-certified driver is usually the most stable. This is best if you are simply concerned with system stability, not speed and performance or taking advantage of special hardware-accelerated features. It is also the driver that's installed by default during system setup, if one such driver exists for your device.
The manufacturer-issued WHQL driver is usually a more recent revision than a matching Microsoft WHQL driver and is usually the most feature-rich. Use this driver if you want to balance stability against support for advanced hardware-level features (such as certain video acceleration functions in the case of a video card).
The unsigned manufacturer's driver is usually the fastest or most optimized. It not only supports all hardware-accelerated features but also may use optimizations that have not been WHQL-tested. If you are concerned with stability and the integrity of a system build, you should probably not use unsigned drivers at all. They can be useful for isolated system tests, but using them in a production system image is probably not a good idea unless you have no other choice.
Please note that some system drivers have no direct manufacturer equivalent. The Universal Serial Bus (USB) Audio Device driver, for instance, does not seem to have any manufacturer substitute.
Serdar Yegulalp is editor of The Windows Power Users Newsletter. Check it out for the latest advice and musings on the world of Windows network administrators -- and please share your thoughts as well!