Antivirus8 need immediate scan HELP!

for the past 2 days i have been getting this weird popup saying i need to scan my computer. it just pops up randomly, once when i was on facebook and ebaums world. its a rogue but what i cant understand is when i scan with nis 2009 it says im clean. today it popped up again. what should i do? am i infected? its called antivirusscanner 8 i believe