Video Editing problems!?

I have tried a couple different video editing programs (adobe elements, windows movie maker, avs) and they all have the same problem. I can play video fine, but as soon as I go to edit the video (Clip, Trim and Splice) then the video will stream VERY poorly. This makes editing near impossible. What is causing this? Is it my computer? It is an older computer (Sony Vaio 2004) but has upgraded ram and core so its pretty fast. Any ideas?