Cannot boot from CD/DVD

Hello,
I am Abdul Halim. I am using this computer configuration:
motherboard:Intel g41
processor: core 2 duo
HDD: 320 gb
Dvd R/W: Asus 24x

I was using Windows xp sp3. recently I have installed windows 7 in different partition from DVD successfully. but after that I have facing problem when I want to setup windows from CD/DVD/USB. Then I format that partition(where windows 7 was installed). but I was still having problem. then I installed windows 7 in the partition where windows xp was installed and remove windows xp files. now this computer is running good but does not boot from CD/DVD/USB. why this is heppening?
Please help me to solve this problem...