If your computer has an AMI or Award Bios, the manufacturer can usually be identified from the Bios string displayed on screen at boot up, usually in the lower left corner.

An easy way to do this is to Download AMIMBID, it's a very handy little Windows utility from AMI which automatically decodes Bios strings and lists the version, motherboard manufacturer etc.

Although designed for AMI Bios it works on some other Bios too  All versions of Windows.