We are facing similar kind of issue with Lenovo - Z560, B450, G560 models. Once applying encryption the system gives error message. (EEPC 5.2.5)
Disabling mouse, Stargin Operating System.....
We are not able to install AHCI driver with XP OS as well with above procedures.
Kindl let me know if you have any solution to that perticular laptop models.
I branched this off as the original thread was about a specific Dell problem, solved when Dell fixed the bugs in their BIOS.
Your problem though could be simply that your MBR is flawed, or that you have a root kit virus on the machine already, or that your boot drive is not a standard MBR BASIC disk - you need to check the basics first.
You should also of course upgrade to the latest version of product, and check you have the latest BIOS version - lots of changes to support buggy BIOS in the last year.
We had this happen on the older version of EEPC with Eseries Dells. The fix was to use the feature "update number of sides reported in the OS boot sector" and make a fileset specifically for them. The Eseries has been the only real laptop that we had to do this on.
That option fixes a non-standard MBR/Boot sector, so again, it's something you would have inhereted from the Dell factory image. I'm glad the option solved it for you though - it should rarely be needed nowadays though.