ACHI and network boot problem.

Hi friends, i had a strange problem with my laptop.. i.e, all of a sudden the hard drive changes to ACHI and network boot is kept as a first boot. As ACHI is selected, windows crashes with a BSOD. Now, i have manually reset the boot order and also changed the sata to compatible mode..

But this happens again and again but NOT ALWAYS.
I cant figure out what the heck is wrong??
I have the latest drivers and a paid version of k7 antivirus with the most recent updates.
my laptop is a thinkpad with windows 7 ultimate installed..

Please help..

TIA