To raise it to 13 we can use Priority boost in sp_configure .
But reciently , I got a question if we can set the CPU priority to lower than normal.There is no option for this inside SQL Server.
So there was in inderect answer to this : use affinity mask , so that SQL is restricted to particular CPU/CPUs .But thats not the correct answer .
So is there a way to do that via task manager .Well , it has the option but we cant as we get some access denied error
What you can do is :
1) Open the DOS window >>Start /Low "
2) This will open another window .
3) In that new window type : "C:\Program Files\Microsoft SQL Server\MSSQL.2\MSSQL\Binn\sqlservr.exe"[path of sqlserver executable for an instance] -sMSSQLSERVER <--instance name [copy it from the services dialog box]
4) connect to the management studio or OSQL/ISQL.
5) cross check in task manager .You will see that SQL Server is running under low prority
6) Now from the task manager you can change the priority ...not sure why :) ...
Hope this helps .......