How to scan in the computer?

How come my computer says that it's at risk and I should run a scan when I JUST got done scanning?

• Okay i got home today and my computer said there was one threat detected. So, I got rid of the threat, it said i was all good, green and everything meaning im okay. It said that i was protected and all. So then like 5 minutes later the computer icon goes from green to yellow and says "You havent run a scan in a while. This could put your computer at risk." with a scan now button. So i scan, it says it scanned a ton of items and no threats were detected....And then immediately after it once again says "YOu havent run a scan in a while." whats the deal? can someone please help me?

• Answer:

  what sweeping program do u have? Buy an accredited Anti-viral/Anti-malware software, i reccommend Webroot. Microsoft Security is just a basic, it doesnt cover everything. i went through the same thing and i bought webroot, its worked puuurfect evr since.