When install win7 ultimate into dell inspiron n5110 shows blue screen

When you are asked to install drivers, press F7. This will disable the ACPI check.

STOP: 0x000000A5 (Parameter1, Parameter2, Parameter3, Parameter4)
The ACPI Bios in this system is not fully compliant to the specification. Please read the Readme.txt for possible workarounds, or contact your system vendor for an updated bios.

The bios in this system is not fully ACPI compliant. Please contact your system vendor for an updated bios. If you are unable to obtain an updated bios or the latest bios supplied by your vendor is not ACPI compliant, you can turn off ACPI mode during text mode setup. To do this, simply press the F7 key when you are prompted to install storage drivers. The system will not notify you that the F7 key was pressed--it will silently disable ACPI and allow you to continue your installation."
The parameters Parameter1, Parameter2, Parameter3, and Parameter4 vary from computer to computer.

http://support.microsoft.com/kb/314830

Hi  

The above only works with a GENUINE LEGAL copy of Ultimate....and of course with a legal copy you can always ring Microsoft for support  

All the best Brett