Bootup Glitch

Hi Folks, I am new to forums so, please, bear with me. I have a Mac Mini and I was installing NIK software plugin to aperture. Everything seemed to go okay until I shut down and then started up, later. The machine starts to boot-up and gets to then constellaion screen and then starts to shift back and forth between that screen and a brief white screen. Finder is not loading but I can get on the internet with Safari. Aperture alternately is trying to load. To be perfectly honest, I think I messed up. I was supposed to shut down all aps before installing the NIK software. I got distracted and forgot to do that and loaded the NIK software with other aps active -- I think. Any ideas on how to straighten this out. I am a bit of a novice and live in an extremely small town -- far away from any tech support (40 miles). Thanks for your help, Jack