The menu on the right has links to most hardware related pages here. The DMOZ directory is a good place to find much more hardware information.
Don't buy a piece of hardware before checking its compatibility with your system. Does your version of Windows support it? Do you have the right interface (USB-2, Parallel, FireWire, etc.)? You might be able to determine this at http://www.microsoft.com/hcl/default.asp .
If you add hard drives, video cards, or other power consuming accessories, you'll need to take cooling requirements into account. [measure]
You can find all kinds of information about computer hardware on the Internet -- hard drives, and hardware upgrades, for example. I got some good links about scanners using the keywords [tutorial scanner guide]. The keywords [scanner review] turned up more results from another perspective.
There are dozens of top-rank suppliers of everything -- software, components, accessories, peripherals -- to do with computers. My long-time favorite is NewEgg. I also use the Amazon.com Electronics section. Their prices are often very competitive. When I buy things online the key words for me are "trustworthy" and "customer service". I can vouch for both NewEgg and Amazon.com. Others vouch for TigerDirect.
There are many other good suppliers, but it's tough to tell them from the problem ones. Many comparison shopping websites have vendor ratings that may help. I'd compare suppliers with ratings at more than one of those sites before buying anything important.
Remember when the Sears catalog had three choices -- good, better, best? These days, the wide variety of choices for almost any hardware item makes it hard to select the right one for yourself. Here are some places you can find some reviews that might help.
"Drivers" are simply the code (software) that the operating system (Windows) uses to control the hardware. There are drivers for sound cards, mice, modems, monitors, CD drives, printers, cameras, and most other hardware integrated in the computing system.
Some drivers have bugs to get rid of. Others need updates to add new functions. You can usually download updated drivers at the manufacturer's website -- if you can find them. :-) The drivers page has more information.