It can be a few things but usually the atapi.sys file is corrupt. If you can boot into your desktop, open the "run" box and type in "cmd". In that box type: sfc /scannow and press enter. There is a space between sfc and /scannow. You will be asked to put your Windows CD in the CD drive.
When did this problem begin? Have you installed a new harddrive recently? Have you installed new memory recently? Remove and re-insert the hardrive cables. Remove and re-insert the memory. Be careful about electro-static discharge. Sometimes just doing that will fix a bad connection.
Sometimes this error is caused by bad memory. You can download Memtest86 here: http://www.memtest.org/
Grab the prebuilt ISO.zip After downloading the file right click it and select "extract all" Use Img Burn http://www.imgburn.com/index.php?act=download
to write the ISO to a CD. Keep the CD in the drive and reboot.
Also, there could be a problem with the hardrive. if you know the harddrive manufacturer you can usually go to their website and download their hardrive diagnostic software.
Sometimes erasing the hardrive (absolute last resort) will solve these kind of errors. Just be aware that you will lose ALL of your data and it mat not solve the problem. Darik's Boot and Nuke works good for that: http://www.dban.org/