Hi. I agree, Krunner ist wirklich cool:
10 Awesome Features of Krunner in KDE 4 - Make Tech Easier
You're sure it's Krunner which is thrashing your disk? In my case it's Strigi. It keeps my disk busy for a while after logon, but only since I installed KDE 4.3. Everything was quiet in KDE 4.2.x.