First - use background rendering to handle the toughest areas with lots of layers (Second last checkbox in the 'Layer Settings' list in Control Tree)
Then do these steps:
1- If you go into task manager and find out what is using all your CPU speed and Memory, you should be able to minimize the computer's multitasking so it's just serving SE.
2 - If you're going to be using just SE for extended periods, you can even close down explorer and re-start it from Ctrl Alt Del Task Manager when you need it again, because it uses CPU and memory.
3 - Try making your video output window smaller. Full screen will jerk when quarter size will not, and even smaller is smoother.
4 - Right click the video output window and try different fielding settings for smoothness.
5 - In the Preferences Window there is an option for video output monitor high or lower res (Can't remember the exact wording but it's pretty obvious) and that makes a big difference.
6 - If you really want to go to town, try uninstalling any software you don't need, defragmenting your hard drives, and disabling any virus scanning software (Which will be one of the things you'll find in step 1)
The codec can also be a factor. Uncompressed will probably cause playback problems. If you're sing Newtek's codec (SpeedHQ), it will preserve a lot of the quality, and should play back faster (or smother) than most codecs.