Sometimes my computer will boot, sometimes it won't even turn on. What's The Deal?

Okay, so here's my specs:

Dell Inspiron 531 mini tower
ASUS M2N61-AX Custom Dell Motherboard
AMD Anthlon x2 2.3 ghz dual core processor
Nvidia integrated graphics
2 GB DDR2 RAM
Seagate Barracuda 7200 rpm 500 gb hdd SATA II
230 watt PSU
dvd rw optical drive
Windows vista home premium 32 bit

This began four days ago. Recently I've been having issues getting my computer to boot properly. I would hear the computer and fans start up, but would get a blank screen with no HDD activity light. So, I'd just turn off the computer by pressing the power button and try to turn it on again. Sometimes it would do the same thing, other times it would boot if I was lucky. And a few times while my PC was running, the screen would freeze and turn into a bunch of zigzag lines. This would force me to turn off the computer manually by pressing the power button. At first I thought, "Well, my hdd is just failing". But later, after I took off the side panel and cleaned a little dust inside the case thinking maybe that might solve the issue, I pressed the power button to turn it on again, and it wouldn't even try to start. The fans would just spin for a few seconds, then kick off. The only way to get it running again was to go to the back of the case and to flip a little red switch (I'm assuming this is like a reset button on the PSU?) But I'm not sure if resetting this had anything to do with it or not. So, then I'd press the power button repeatedly several times and eventually the PC would respond and turn on. But every time I shut it down, it doesn't want to start again. I even tried putting it into hibernation mode and after trying to turn it back on by pressing a button on the keyboard, the light on the power button indicating it was in sleep mode would just turn off, and obviously kill the computer, requiring me to start it again. After this procedure, it would take a long time to boot, and it told me "resuming windows", then boot right to the desktop where I left off without even requiring me to enter my password like I usually do, even if I had unplugged the power cord. So guys, I need your two cents on this. What do you think the problem is?