The problem could be either:
A.) Not enough memory availableBefore you start watching videos or playing your nascar game, close down any programs that you have running, disable antivirus, etc.... See if that helps.
Your computer will always run faster when there are less programs running because, more memory and processing power are available.
Also, how much RAM do you have? (Right Click on "My Computer" / "Properties")
If you have less than 512 MB of RAM, that could be the problem. ...However, if it was working fine before, chances are that this isn't the problem.
If you could... Download & Install HijackThis and post a logfile. I could probably give you a better idea as to which programs might be causing your computer to be slowing down and, what you might be able to do to speed things up.
Download >>
www.merijn.org/files/hijackthis.zipB.) Corrupt Registry SettingsThere are some fake software/video files out there (usually fake "Div X" videos on P2P networks) that have malicious code embedded in them that delete/change things from your registry causing music, videos, and certain games to run very choppy and, eventually go into "not responding" mode.
What I would do is this...
* Create another user account in the Control Panel (Start / Control Panel / User Accounts)
* Make the new user an Administrator
>> If you use private folders... Make sure that your games and videos are somewhere on your harddrive where all users can see and access them
* Log off
* Log on to the new user account that you just created.
* When everything loads in, close down any unnecessary programs.
* Then, try to play/watch your games and/or videos.
C.) Another program changing/updating priority classes every few secondsYou mentioned changing priorities... are you using a program other than Task Manager to change process priorities?
There are some programs that say they "speed-up" your computer, when in reality, all that they're doing is changing the priority class of a few processes to "high" every 2-3 seconds... Something like that would definitely cause games and videos to run choppy... Almost exactly like how you described it.
D.) Spyware or VirusI'll be able to tell from your HijackThis log if this is the case.