An Atapi sys blue screen crash can occur when the atapi.sys file is infected with malware. If this is the case you may also notice that your online searches are getting redirected.
Getting a PC expert to fix the problem for you is the ideal solution, but if you plan to have a go yourself, ensure you at least make copies of important data on an external drive. Furthermore you should avoid going to sites that require passwords or personal information.
Be sure to run a full scan with an antivirus and anti-malware program to remove any malware present. Of course no program is 100% effective at removing malicious attacks on your computer, so it's a good idea to run several different ones. Spybot S&D is a good one to use and it's free.
If a scan reveals that the file is infected, don't delete it. Deleting it is likely to cause more problems, since it is one of the first drivers to load on boot up and is always in use. Instead send it to the vault/chest until further investigation. It's not that uncommon for antivirus software to throw up false detections now and again.
There have been several widespread cases where updating Windows has triggered the atapi.sys blue screen. This is not because updates have contained the infection (Microsoft aren't that stupid) but because of existing infections. This is just as well, since you may not have even realized you had an infection in the first place.
If you suspect that an update has caused recent blue screen errors, you can remove it through the recovery console. But this won't get rid of the infection. Replacing the infected atapi.sys file is perhaps the best option, but it's recommended to have a professional do it for you. Once the file has been replaced, scan your computer for any malware before you install any updates.
Malware isn't the only cause for atapi.sys BSOD's. CD emulation programs such as Daemon Tools are also potential causes. If you have any virtual drive programs installed, try removing them and see if the problem goes away.
This post does not cover every possible cause and fix for the blue screen error. If you are going to make any changes to your PC, ensure you have backed up your hard drive first. You are responsible for any changes that you make to your computer.