I had built a pc that worked for a week, now it won't post. what is the minimum parts to make it post. Im trying to eliminate whats causing it so I removed mobo from case along with power supply. used 1 stick of ram also have a video card inserted. pluged all power cables and monitor and a keybord. I was going to use the screwdriver on the PW swith pins on the mobo but after turning on the switch on the back of the power supply it powered on. Is it supposed to do that? I had neverhad this problem before and I have built plenty of stable computers in the past. Also first time trying the mobo out of the case thing. also no beeps. does that mean bad mobo. ?

I think you can POST with the following as a bare minimum:

mobo
power supply
video card
CPU
RAM (not sure about this one ... might be able to leave it out)

I've only built one PC but I was able to POST after installing all of the above.

Possible cracked MOBO. It doesn't happen to be an ASUS is it?

It must have some ram.
edit
DDR ram is populated 4-3-2-1 if the ram on #4 goes bad it won't post.(had this happen)
CPU over heat, They burnout fast as in seconds. won't post.

if the CPU burns out no alert will sound. Bad ram I'm not sure if it will alert.

Usually it will alert if there's bad memory or something is wrong with the video card.  If you are getting no alerts at all check the mobo manual for what the possible scenarios are because it can vary by manufacturer. Most likely mobo/power supply/CPU issue.

Does the CPU fan spin when it is turned on?

cpu fan does spin when turned on . if everything powers up should be a good psu? is it suppose to beep if the cpu is bad. Iknow i seated it pretty good. if I remove the cpu what should I look for to see if it is bad. thanks guys. what s a good way to see if mobo is bad.

Did you install the heatsink on the CPU yourself or was it already attached?  A bad installation can fry a CPU.  It can be difficult or impossible to really tell if the CPU is bad just by looking.

Try clearing the CMOS as per the manual, usually involves switching a jumper, disconnecting the mobo from the PS, letting it drain for a few seconds, and reconnecting.