Why does Nav or NIS 2010 throttle my hard drive when scanning?

One other question,

 

 

Whenever I do a scan, even minimized, both Nav and Nis really slow my browsing and other file operations way down. Is there a setting to have it run at the lowest priority than what is in the program, as I dont wish to run the scan unattended.

 

Tnx again,

 

Alex :)