Asalam o Alekam

i have a problem main ne apnay desktop pc main windows 7 install ki hai ab obara xp install karna chata hon lekan plz wait installing windows par aa kay Blue screen aa jati hai jo eror deti hai keh plz chek your newly installed driver or run a virous scan disk jab keh agar dobara windows 7 ko karoo to ho jati hai windows 8 b ho jati hai main ne cd se dvd se usb se har tarah try ki hai xp nai install ho rahi bios main ja kar sata configuration ko b AHCI st ATA par kia hai lekan same problem aa rahi hai agar kisi ko pata ho to plz is ka hall batayee detail kay saath tnx in advance

pc specification.....

Dell optiplex 960
2.8 core 2 due
4gb ram
500 gb hard drive
z help if any one knows about this problem .....