Computer won't start up properly

I've been having this problem for a while, and I kind of abandoned my computer... until I realized it was the only comp w/ a capture card in it. I tried turning on again today, and almost every time I try to turn on the computer, it does not boot properly. Instead, it greets me with really loud fans and a blank screen. When it does boot, I'm lucky if it stays on for a minute, because it usually freezes within a minute of me logging in. I've tried taking out what I don't need, but I haven't tried replacing anything. I'm don't have spare graphics cards or RAM sticks to do that. Do you guys have any suggestions? (that don't involve taking out essential parts of the comp)