Do you remember when you first got your computer, how fast it was! It was great! But little by little it got slower and slower and you don't know why!
There are several reasons why your computer can be slow. Especially when it happens suddenly, then it can be slow because of malware (spyware, adware, trojans, viruses..). However, it's not always malware causing this. A computer that suddenly becomes slow may have a virus or spyware program robbing its resources. You should run a virus scan and spyware sweep as a first step in diagnosing a suddenly slow computer.
PCslow Doctor will give you a good start at diagnosing and correcting issues that slow your computer down.