avi files play smoothly on Windows Media Player.
avi files play not so smoothly in Adobe Premiere Pro - why?
Can I download a more efficient codec?
Can I change a setting somewhere in Adobe Premiere Pro or in windows to improve this?

My dektop machine spec:

Windows XP Pro
1GHz Pentium III, 133MHz FSB
384Mb RAM
brand new 120Gb 7200rpm Samsung Spinpoint IDE Hard Drive connected to UDMA66 motherboard
QDI Advance 9 motherboard

Yes I know it's not the latest spec - it's an old machine - my Dell laptop has no problems. But this old desktop is going to be lent to a friend.
I know... I KNOW!!! (see story below)

As I say, AVI on the desktop plays smoothly in Windows Media player, but is not so smooth in Adobe Premiere Pro - how can I remedy this?
Why is it a little jerky in Pro and smooth in Media Player.
I think it must be that the codec in Pro has more overhead than in Windows Media Player - but I don't know why. Wonder if I can download an improved version or alter some settings.

On my Inspiron laptop it's absolutely fine in both Pro and Media Player - smooth playback on both. So I guess the problem manifests with my slower desktop machine. It's more noticeable on my slower desktop machine. But how can I remedy this?

Don't ask me to change the hardware - this has already been discussed to death in the hardware forum - see threads:

The background story as to why I am doing this:
http://www.videoforums.co.uk/viewtopic.php?t=4643

The hardware spec:
http://www.videoforums.co.uk/viewtopic.php?t=4714

I would say your 384 MB of RAM is the problem. For any video editing you should not have less 512 and preferably 1 gig or more. I would venture to guess that premiere is hogging up your memory causing the playback to be jittery.

But since you don't want to change the hardware, maybe you could reinstall premiere. But I still say it is the memory. In video editing the higher the memory the better.