|It depends on what type of drivers files you have saved in your flash drive.|
If you have driver files with .exe extension, you double-click on them and follow the on-screen instructions to install drivers. ( Sometimes, double-clicking the file with .exe extension extracts the file to a folder in a location you indicate. Just open the folder and look into it. If you have a SETUP.EXE in the folder, you double-click it and the drivers start to install. If there are files with .inf extensions, follow the steps below. )
If you have driver files with .inf extension, you go to the Device Manager and open properties of any device for which you have downloaded drivers. Click Driver tab and click Update Drivers button and point to the location of the driver files, in your case, the flash disk drive.
ONE WORD OF CAUTION: ( If you have already installed XP, disregard this caution. ) Most notebooks that come with Vista pre-installed have SATA hard disk drives. Windows XP does not recognize SATA hard disk drives natively. So you cannot install Windows XP readily.