8400 GS should be able to run TF2, not well mind you, you'll have to crank down all the settings, but still playable.
As for the texture/icon problem in the pic you posted, I used to get that every now and then on my old PC with an 8800GT, It couldn't have been an overheating problem cuz I had the zalman cooler version and It would run about 40C.
What are the rest of your system specs? what power supply do you have (wattage, brand, age)?
my computer is an acer, got it 2-3 years ago, or maybe later than that. AMD Sempron processer 3300+ 2.01 GHz , 1.18 of RAM, and how do I check for my power supply??