Note these tools may also find driver updates for devices other then your webcam. Connect your cam before you run the tool. Update: In addition to the manual search method described below, here's two FREE driver finder tools you can also try that may help find your driver. (For example, an older web cam may only have XP compatible drivers.) But you won't know unless you first try and look!
Note that older cams may not have drivers that work with newer versions of Windows. The driver must also be compatible with the version of Windows you're running. This guide describes how to identify the Hardware IDs for a USB web cam, then try and find a driver that matches the Hardware ID. Windows Device Identification strings look rather cryptic (for example, USB\VID_093A&PID_2468\5&B114931&0&1) but these strings are fundamental to Windows Plug-and-Play and how Windows figures out which drivers work with what devices in the first place!