As a conclusion:
I upgraded Realtek driver for onboard audio from Gigabyte support page (http://www.gigabyte.com.tw/Support/Motherboard/Driver_Model.aspx?ProductID=3005&ost=xp#anchor_os) and then re-enabled onboard audio.
The PC runs OK and sound is OK too.
So, for GA-MA790FXT UD5P owners encountering BSOD at login (XP SP3), I recommend the upgrade of Realtek driver, in the case onboard sound is enabled.
Thanks for the valuable information I found here.