help--computer running extremely slow

My computer has recently been running extremely slow, to the point of inoperable. The odd thing is; it runs 100% normal for 5 minutes after I turn it on and THEN it runs slow. These problems include inability to play audio/video files, mouse lag, keyboard lag, etc.

System restore to before the problem occured does nothing, neither does antivirus, ccleaner, and spyware removal programs.

The cpu usage rate is 100% with no processes or programs running in task manager, which makes no sense.

any help would be appreciated.

Are both fans running? It might be getting hot after 5 minutes.