Chances are that it is likely software, I psu units that are nearly as old as I am that still run without any issues. Get ccleaner to rid of crap saved by the os and browsers as well junk that has collected in the recycle bin. Wipe any old and unwanted os restore points as it will free up a Lot of space. Last but not least give Comodo (very good anti virus) and Lava soft Adware a shot to rid of any tracking cookies and malware. Malwarebytes also does a vry good job cleaning up infections that harm system performance. Last but not least free up space or upgrade the ram. Hard drives typically start losing performance once they are more than half way full or have been used for a very long period of time ware the free space wasn't wiped.
Now for the tough questions. How old is the machine? What type of cpu (Intel, AMD) ect? How much ram and what size and brand is the hard drive?
I run Ccleaner daily. i run antivirus weekly. I defrag every month and have wiped all useless programs and junk that are on it. I added an extra hard drive for multimedia only to alleviate use of the main HDD. The computer is pretty old. Here are the specs. http://h10025.www1.hp.com/ewfrf/wc/document?docname=c00.... I also run Malwarebytes every week so I don't think its a virus related issue.
I upgraded the ram to 1gb the max the system can handle. It might be Firefox that crashes on videos because when I use Chrome or Safari to watch videos I don't get this problem. I did notice that when I played Sims 2 it ran it perfectly and also Harry Potter ran flawlessly as well so I don't understand why Firefox has this issue. I tried to play Gunbound because I had played it previously,but it would not work properly. I got vertical lines running through it. Thanks again everyone for the help.
The card simply got hot and it will happen if you push it at full load long enough. Just check the fan if it does have one to see if it is working. I had a 8400gs my self and they are weak but oh well but just be sure to keep it cool or it will act like any other card and show artifacts.