I use LottaNZB to download from usenet. LottaNZB is a GUI for HellaNZB. HellaNZB downloads all files and calls par2 and unrar and leaves just the files you want. Easy. But.. when I download a big file, like a DVD image, and the download is finished the processing begins. And this will sometimes sloooooooow down my computer. Sometimes it's frozen.

When it's frozen I'll have to wait. When it's not frozen but just slow I can change the priority of the processen and give them a higher nice value, which is in fact a lower priority.

Now my question: can I somewhere set a default value for processen that must always be low priority, like 7zip, unrar, par2, hellanzb?
So I don't mean starting unrar with 'nice unrar 10' , because I want unrar low priority whenever it's started by any program or by myself.
So can I set a default priority for a certain program?

Or another option: which processes I have to give a lower nice value, thus higher priority, to prevent my system to freeze.
Which processes are responsible for screenupdating and mouse interaction?