Net Framework has nothing to do with the make or model of your motherboard.
You can download Net Framework 4.0
from Microsoft here: http://www.microsoft.com/en-gb/download/details.aspx?id=17718
For Windows XP, you must have Service Pack 3 (SP3) installed, otherwise Net Framework 4.0 will not install.
In case you also want drivers for the motherboard (they have nothing to do with Net Framework) they are here:http://support.asus.com/download.aspx?SLanguage=en&p=1&m=P5G41T-M%20LX&hashedid=TOOAPqOMZFVMI9Ji