I have a dell demension 4700 and it won't start i push the power button all that happens is the power light blinks once.

More than likely you need a new Power Supply.

http://www.techspot.com/vb/topic53866.html

If you have a spare Power Supply lying around, install that and see if that's your problem. Which it most likely is. Post back with results.

i have extra power supplys but this power supply is 305 volt and i don't find those often. I guess i would have to buy one but i'm not sure if that is the actual problem.

305V? I've never even heard of that before, only 110/240V @ 60/50Hz. What country are you in?

No country has 305V power supply.

Obviously a typo.

Must be. He must mean 305W. If that's the case, you don't have to have the same power PSU as your old one, as long as you have one good enough to power your PC.

E.g. you oculd buy a 1KW PC if you wanted to, but it would only give your PC the power it needs.

305 W is a pretty odd figure.

Goes againt the principles of engineering.

Probably is 300 W or 350 W.

I checked it is 305 W.

Ok. If that is what it says.

Is this written on the PSU, or a figure from some s/w ?

what is s/w

s/w is short for software.

it's on the psu.

It would only be written on the PSU because all software can tell you is what voltages are coming out of the PSU.

305W is a random number, but PSU's are like that. You can get 385W as well. You could pretty much get any number under the sun (so long as it ends in 0 or 5 )

then what should I do?

what wattage would be sufficient to power the computer.

I can't tell you what wattage you should get because I don't know your specs, but if you go here and input your specs, it will tell you about how much you need, but you will want to add 30% on top of what it gives you.

i'm not sure what some of this things:how do know what rpm your hard drive is and what kind of processor i have? what is compacitor aging and what if the video card i built in the mother board?

how do know what rpm your hard drive is
unless you have a laptop, it will be 7,200 rpm. If it's a laptop, you will almost certainly have 5,400.
If you have a SATA hard drive (which you will have if you bought the PC in the last couple of years) just select the amount of drives that you have. You can check this in My Computer.

what kind of processor i have?
Right click on My Computer and click Properties. On the left hand side it will tell you what CPU (Processor) you have and how much RAM.

what is compacitor aging
set this to 15%

what if the video card i built in the mother board?
If you don't have a built in nVidia chipset, select the first option "32mb or less AGP/PCI Video"