Exclamation mark on start up

I was playing on my computer that I built 6 years ago, when it went into a random blue screen. The first time i started it up after the blue screen the normal black boot up screen was covered with equal signs, but all other screens were normal, as in asus motherboard screen/windows startup. However when i load into windows it was stuck on a 800x600 resolution. I reset the computer and then the equal signs were Exclamation marks, any ideas to what might cause this?
I have a Asus p6t mb and a 9800 gtx+ graphics card