I really appreciate you responding to my post with suggestions of help for my computer. I was able to solve the mystery and I am happy to say that it is back up and running again! Long story short, I bought a barebones kit online a couple years ago that contained a Gigabyte motherboard, an Asus chip, a DVD-RW drive, a Seagate 1.5TB HD, 8GB of memory, and all the other components needed to put it together, including the case. When it came, I put it all together, never noticing that the CPU fan that they supplied in the kit was a three-prong, instead of a four-prong, but as I never noticed, I put it on there anyway & it started right up and worked fine for about 6 months. Then I started having blue screens & other probs., including it shutting down unexpected, but that time it turned out to be the memory. So I replaced them & it was fine again until about six months ago when I notice the HD start to make a clicking noise. Anyway, I replaced the HD last week but the computer kept shutting down, & now I know it was due to the improper fan, as the fourth prong on the "right" fan is what communicates with the comp. (bios) & tells it when its getting too warm. So I went out & bought the correct type of CPU fan, changed the settings in the bios so now it will set a buzzer off if it gets too warm...and voola...it works perfect. I hope explaining this helps others who may have similar situations. And thanks again for your offer to help!