Trouble with streaming video

[Max Raker]

I have a problem where after a certain amount of time my computer refuses to play online videos like on youtube. The sound stops completely and the video stops 2 seconds after I start it. When I reboot it works fine for a bit and then does the same thing. I can still play Mp3s and videos that are on my computer always. I am using firefox but the same thing happens if i use IE.
I forgot to say that I am using XP.

Thanks.