I also had this symaddln error occur on windows vista for my media player. It gives you a box that says do you want to disable symaddln and you can either opt to put a check by the name symaddln or not. Dont disable it by putting the check mark by the name. Simply click ok to acknowledg that you have seen the message saying the program doesnt want to run and then shut your computer down. Then turn your computer back on and go back into the media player and see if you still get the same message saying that the program wouldnt run. I did all this and i have never had the message again. I can watch movies or whatever. It never sends me to the desktop and there are no problems whatsoever.
If you clicked yes to uninstall symaddln by putting the check mark next to the name then i am not sure what you can do. You could try and re-install it by searching for it but they say it is hard to find in a search so i am not sure how you would do this but my knowledge is also lmitedwhen it comes to computers. I just knew to do this by playing video games when younger. When ever you get an error on a gaming system it is almost always fixed by turning it off and then restarting it. This worked for me with the media player too.
So in summary if you didnt disable it click the ok box without checking the symaddln icon icon and shut the computer off. Then turn it back on.
If you did disable the program try and find it an re-enable it and if it shows that error again just click ok without checking the symaddln icon and then shut down the computer. Then turn it back on. Problem should be solved