Hi-I've got audio/video problems I hope someone can help me with.  My computer is getting old but, I've taken good care of it.  It's a Dell Dimension 3000 that I got around '06.  I replaced the monitor around '09.  Running XP SP 3 and using Chrome right now.  A few weeks ago, when I'd be watching videos on Youtube, the audio and video wasn't in sync.  This would happen intermittently at first.  Then I noticed that, if I was watching on full screen, it was alot worse than the small screen.  It also SEEMS like the first video or 2 I watch will work pretty good but, the more videos I watch, the worse it will get until it isn't worth trying to watch them.  Today I was watching one in small screen - it was playing fine - and I went to full screen.  For 30-45 seconds, my mouse disappeared and the video froze.  When the mouse came back, I used it to reduce screen SIZE and, during the process, I caught a glimpse of a box that said "video (or program-but I thought it said video) not responding before it went to small screen.  Back at small screen, video still wouldn't play - it just had the little round thing going around in the middle forever.  (Sorry-I forget what it's called!)  I used the Flash Player uninstaller to uninstall Flash Player and reinstall it.  I did that twice. When I replaced the monitor, I thought it was a graphics card problem at first and bought a NVIDIA GEForce 5200 card.  It's been in the computer but not in use.  The Intel graphics controller that came with the computer says its working fine but I switched to the other one after updating both drivers to see if that HELPED but its still the same.  I'll try anything anyone can suggest and would really appreciate the help.  I know this computer is getting old but, I can't afford to REPLACE it anytime soon.  Thanks!This system is likely showing its age. My daughters computer is similar in age to yours and is a Pentium 4 2.8Ghz HT with 1GB DDR 400Mhz RAM and GeForce 6200 8x AGP with 256MB video RAM running Windows XP SP3 and videos played in HD are just like this problem you have and it full screen HD is just about unplayable, small youtube window it plays but frame rate is poor, while if she selects the lower quality playback of the 360p otion in the Youtube window vs 720 or HD it plays better and can go full screen without the audio/video lag.

Looking up the Dell Dimension 3000 it shows that this system is either a Pentium 4 2.8Ghz or a Celeron D 2.4Ghz and came with 256MB RAM, which hopefully your running on more than 256MB, of which I would suggest 1GB for best results.

If you have a GeForce 5200 video card installed you would be connected to that and not the integrated GPU on the motherboard. I would MAKE sure you have the GeForce graphics drivers installed from nvidia and not use the default drivers that windows use.

Also if you are running on 256MB of RAM, I would go with 1GB. This RAM can be bought cheap for good used sticks on ebay.
*My daughter was on 512MB 2 years ago and upgrading to 1GB made for better performance. I found a 400Mhz 512MB DDR stick for $8 on ebay with free shipping back then. According to this info linked here it looks like your system can take 2GB of RAM. http://en.community.dell.com/support-forums/desktop/f/3514/t/19439698.aspx

Also the GeForce FX 5200 is not that powerful these days even for just streaming video. My daughter has the GeForce 6200 which is the next newer series and it has to run on 360p to play ok full screen and video resolution of 1024 x768 ... So if your resolution is set high, you can lower it and increase frame rate. This is a trick to getting games to running on faster frames per second that also works for streaming video. Hi-I am sorry I did not get back to you after you replied to my post.  I did change my screen resolution and that did help for awhile.  Then I got hit with the worst case of the flu I've ever had - was sick thru Christmas and New Year's and just getting back on my feet.  Only made it to the computer a few times and found my video problems had progressed.  I bought a used Dell Optiplex on Ebay today - hope I didn't buy new problems because it has no operating system installed!  I posted a question on the forum in regards to installing Windows 7 a few minutes ago.  When I did that, I was able to locate this post so I could reply and thank you for your help.  I appreciate it very much and many thanks to you!