Sound driver driver detective shows that my pc has no PCI modem and USB device, too. what is it? please help, thank you

This is, again, for your sound card problem -> Open 'regedit' (Start -> Run -> type Regedit and press enter), search for 'enum' (Do this by pressing Ctrl+F on your keyboard, and type "enum" and press "find". There should be two occurrences of "enum". Both will have a subfolder called 'HDAudio'.  Delete both HDAudio folders (one may require changing the permissions on it).  At this point you should be able to install the driver. Go to Control Panel, and click on "Add New Hardware". Have Windows search for new hardware. It should detect the audio device as though it has never seen it before, and install the drivers for it. This is actually a sure-fix solution. I think this may work!  As for the other two problems, viz., PCI Modem and USB device, you have to again download the necessary drivers for your motherboard from the internet. Search on Google with "USB Drivers for G31T-M". And since you are using the internet without any hassles on your PC, do not bother about the PCI modem.