关于windows系统目录下的 driver.cab文件

WindowsXP系统为例子，C：\\WINDOWS\\Driver Cache\\i386\\目录下，有一个driver。cab文件，如果打了service pack，还应该有sp1。cab或者 sp2。cab这样的文件。
我的问题是：这个咚咚是什么用处？
是不是所以通过了whql的设备的信息 包含在这个文件包里边？

[编辑 -  6/16/05 by  dregs]

cab只不过是类似RAR的一种打包文件格式。里面的文件才是重要的

The Windows XP Professional device drivers included on the Setup CD are stored in a single cabinet file named Driver.cab. This file is used by Setup and other system components as a driver file source. You can view the contents of the Driver.cab file by double-clicking it in Windows Explorer.

Information files (.inf files) are searched when Windows XP Professional starts or new hardware is detected. These text files provide the names and locations (typically Driver.cab) of driver related files and the initial settings required for new devices to work. During setup, Driver.cab is copied from the installation CD to the local hard disk in the %windir%\\Driver Cache\\platform directory. (The variable platform describes the architecture of the system, for example, x86.) The folder where the file can be found is specified in the registry entry DriverCachePath in the subkey HKEY_LOCAL_MACHINE\\SOFTWARE\\Microsoft\\Windows\\CurrentVersion\\Setup.

具体可见
http://www.microsoft.com/resources/documentation/Windows/XP/all/reskit/en-us/Default.asp?url=/resources/documentation/Windows/XP/all/reskit/en-us/prdh_dmt_ylow.asp

还有一个疑问。
为什么有的设备插到电脑上 会自动安装驱动。
有的设备却会弹出 发现新硬件向导。