I think your Thermal Paste on the CPU HeatSink might be dry. When this happens it like metal touching metal and the computer shorts out so it can't boot. You need to open the case and you will see 4 screws that hold the cpu heatsink on top of the processor chip. There are lots of videos on Youtube that show you how to replace the thermal paste (sometimes called thermal compound). I'm going to post a link here to one. I normally apply a thin coat of the paste all over the cpu and also on the heatsink. Then I put a coat of the paste in the middle of the cpu just like the video shows. This insures that I have coating all over the cpu and the cpu/heatsink. Don't worry as the video says about letting it dry. Put it on and turn your computer back on and start using it. https://www.youtube.com/watch?v=BQgCUcAAdac How to Replace Thermal Compound
Unplug the unit from electricity, wash & dry your hands, take steps to avoid you transmitting static electricity(look it up if you are not familiar). There are several sheet metal screws around the edge in the back of your CPU. Remove the ones that appear to join the outer lip to the inner lip all the way around. The front & back should now separate into 2 pieces. One the shell & the other the bulk of the system. The hard drive is usually located on a standard mounting shelf by itself with 1 or 2 small screws holding it in on the side with 2 wire connections in the back. The larger is a ribbon cable the is used for data & the other is the power cable. pull out the cables, unscrew the screws & the hard drive slides out. Some boxes may have something that blocks your access to screws, if so, temporarily remove them so you can work. If you are not sure of a potential problem...take a smart phone picture so you can see how it goes back together.
Please remove any and all USB devices, DVD/CD's, floppy disks from the PC then power it off completely and try to boot again. If it boots properly it simply means one of those devices is set to be bootable but isn't set up to boot your computer. To identify replace one at a time and reboot until the problem returns. Should removing all of them fail, chances are your windows drive has failed and needs to be replaced and re-installed with windows. Dell's generally arrive with a set of DVD's that can be used to set up a new drive, this will get you running again but you will need professional help if there was anything on the original drive that requires recovery.
The blinking light does indicate power problems here is the exact message: "The power supply is receiving power from the source but is unable to properly distribute it to the components." I would suggest pretty much the same thing. Remove any additional components you can, reboot and see if the problem still persists. If it does I would call back informing that with only the basic componets it still fails. If you are able to boot to windows go to system in the control panel and under the advanced tab unclick the option 'automatically restart on system failure' something to that affect. That would give you an error message that may diagnose the problem more specifically. You would have to verify with dell if you could get a larger power supply for that particular system.
This desktop has a motherboard in it that has an intel LGA 775 processor socket in it. From the spec's I found the cpu that ships with the system should be able to scale up to 3.5 Ghz. But you should be able to replace it with any Pentium 4 that fits into an LGA 775 socket.
This is generally an indication of boot failure , often as a result of new hardware addition or new software installation. Try pressing the F8 key repeatedly (about once per second) while booting and boot in safe mode. If no success , try removing any hardware (especially flash drives) that was added since the last time the machine worked and try the above again