Not able to play online and videos on system properly

Hi guys,
I currenlty have a problem that whenever i try to play videos they are not playing properly ie they play for a few seconds and then stop and then continue....like am not able to play videos smoothly.It may be online videos like youtube veoh on any browser so far i have tried chrome,ie,mozilla,safari but same problem.Even sites are taking some time to open.Earlier they used to open very fast.i installed the latest adobe flash player too but same problem.i tried playing some videos in my system using vlc but they also are having same problem.

My system is dell vostro 1400 and operating system is windows vista.

I wonder what the problem might be?please help.

Try these steps:

1. Check for Malware \ Viruses
2. Try updating your codecs
3. Try defragemtation of your hard drive
4. Download and run a program called ccleaner
5. Update all your drivers.

Hi,
I have used ccleaner.I also used malaware anti virus the result is no infections.But still the problem exists.

Please let me know what to do next.

Thanks.