Does process lasso modify the windows scheduler settings when you install it?
Nope, PL don't change anything with just install and using it(except you open other function that jump out a window and warning you).
If you found it has been changed(default is "Let OS choose"), then it is other optimize software change it.
And
ProBalance is automatic lower the CPU priority for the process, when it can't affect other process/system responsiveness(which don't cause by I/O bound but only CPU usage), ProBalance put it back to normal priority.
You can know more about it.
http://bitsum.com/about_probalance.php
http://bitsum.com/docs/pl/faq.htm
But, for a normal user, just install it and let it do it work. ;)