I be honest I have to side with digitalzom-b if your watching these things online chances are it's your internet connection. If not it's probably the program your using.
That is what I think it is because I imagine you have a prebuilt computer as most people don't list their specs like you did with OS, GHZ speed, and Ram as most will usually list actual parts when they build it themselves. And with 2ghz speed and 2gb of ram and XP Pro that sounds like a computer from a few years ago which is probably a dual core should have enough CPU/GPU power to watch any movie you throw at it.
Which is why I think it is software most likely if your watching movies offline. And if it's from internet connection like youtube, its probably internet. And if it is switch the video quality to lower, youtube for example have 240p, 360p, 480p 720p, 1080p the higher you go up the better the quality and more you have to download to watch it. Or if this is the case open up the video and pause it to let it buffer so you can watch it without the pause.
For what I use to watch movies I use VLC or CCCP (though it's not as refined as others for DVD's but it has always worked without fail), here is a link which will create something that will auto download the newest version and instal it for you (it makes life easy I use it all the time). You can google it and verify the company is legit and doesn't give you any malware, says no to search bars by default, etc.
http://ninite.com/
And if you want an easy way to find out your computer specs heres a program that make it easier as well. Also gives us handy information such as compute temps, etc.
http://www.piriform.com/speccy