I have the same problem when I edit on a PC similarily specced to yours, I just ignore it because when I export the .AVI's or DVD's - they play just fine.

I think its just a function of slower CPU + lower memory + less/fragmented harddrive space.

the end product is fine for me tho so it might be the same for you.

__________________
My System Specs:

Cameras: 2 x Canon XL1s
Computer: Dual Intel Xeon 2.8Ghz processors (hyperthreaded) - Intel SE7505VB2 mother board - 2 Gb PC2100 RAM kingston -Adaptec FireConnect 4300 -2xseagate 10K scsi hdd 150G ea -2xseagate 7200 ide hdd 200G ea - Plextor PX-708A DVD burner - ATi Raedon 9800Pro video with Dual SUN 21\" monitors - Sound Blaster Platinum live!
Software: Windows XP Pro with SP2 - Pyro Pro (adobe\'s premier pro, encore DVD, and audition bundle)