I've often run into this problem as a bench tech. Examine the mainboard carefully for what appears to be a model number. For example "AP5-T rev 3.01". Once you've found one, open your favorite search engine and enter this number, without the revision number. Your results will probably identify the board manufacturer. You can then go to their website for the appropriate drivers. I've had good luck with this method. Also examine the chipset. If it is an Intel chipset ie. 82371AB, you can downloada generic driver from Intel that covers most Intel chipsets. VIA and SiS may also provide generic drivers on their web sites. Lastly, as mentioned in the above reply, check also
www.driverguide.com; if you can identify the board manufacturer, the driver guide can usually point you to a home page. Good Luck.