Machine: iMac G5 iSight, 500Gb HD (240Gb available), 2.5Gb RAM. 3 years and 4 months old. In the last 4 months or so I have noticed the machine has generally slowed down and this is most notable when watching video in iTunes and separately via Quicktime. Nearly every video (no matter what size or quality) will stick and playback will be jerky. The only software I have added in the last 4 months was the MacUpdate bundle in January that included VirusBarrier X5 and...