Sounds curiously like a virus - can't remember where I read it but apparently there was one going around that would cause the machine to progressively slow. May be the cause.
Otherwise I can suggest a full clean install. Right back to a zeroed HD. I can also suggest you check that your RAM is still all there - sometimes it will burn out and your running on 4GB. Also, if you can - get more RAM!
Sounds curiously like a virus - can't remember where I read it but apparently there was one going around that would cause the machine to progressively slow. May be the cause.
Otherwise I can suggest a full clean install. Right back to a zeroed HD. I can also suggest you check that your RAM is still all there - sometimes it will burn out and your running on 4GB. Also, if you can - get more RAM!