Beware of the motherboard revision.
On their site,
http://www.asus.com/products.aspx?l1=3&l2=11&l3=185&l4=0&model=515&modelmenu=1
ASUS claims that "Only PCB R2.0(or higher) support Intel® Core™2 processor"
So, check your mobo for the PCB revision.
If it is something below R2.0, then bye-bye Core2Duo, and you will have to upgrade the motherboard.
If it is R2.0 or above the do the following:
Download the latest BIOS, copy it onto a USB Stick, reboot PC and, during the POST, press ALT + F2 and enter the BIOS flasher. From there you should be able to flash the latest BIOS from the USB stick
Regards,
Dan