HOW TO: know virus in my computer

Sometimes we don't know or not realize virus was effected our computers. This is the way how to knows:1. PC operate slow than before-- update AV, run in safemode, clear your temp file & internet file and scan your HDD2. Download somethings, when you click on the file, nothing happen.-- update AV, run in safemode, clear your temp file & internet file and scan your HDD3. Task manager cannot use--