I cannot start my computer, unless i reset my Cmos Data everytime, if i start my computer, cdrom strarts, hdd strts, but the screen is blank.i can only turn it on if i reset the cmos data, ant that realy annoys me.how can i fix it?

That is a direct sign that the onboard battery is dead or close to it.
Replacing the battery should clear it up.

Open the computer and locate the battery and carefully remove it....make sure that you see which waythe + (stamped on battery) is facing (up or down) Replace with the same number or replacement .

If you have trouble locating the battery or do not know what to look for, please refer to your computer manual.....downloadable PDF file from the manufacturer's website.

most are flat silver objects about the size of a US nickle and most are located towards the far end of the mobo away from the CPU chip.

You can get an idea of what to look for.
cmos battery - Google Image Search

Thanks ill try that,and btw i know how does that batterty look like

this doesn't help

A new battery didn't fix it?
The next step would be to set the BIOS to defaults and disconnect all hardware, see if it gives you the same error. If no error, re-connect one by one until you get the error again. Then you will have diagnosed a bad piece of hard ware.

The only other options I can think of is that the Bios has corrupted itself. The bios is what is programed to start the initial commands for your computer to start up. meaning that it controls the graphics card to start when the computer is turned on so that you can see what options are available. If the graphics fails to initially start until you reconfigure the Cmos settings then it has something to do with this option or the graphics card itself is not performing properly....sorry but I just can't see any other options given the information that you have given.

Try to update the graphic drivers first and then if that doesn't help try switching to another graphics card if you have one.

I normally do not recommend a BIOS update because it is easy to ruin a motherboard....but in this case I think it would be the next option.

There are a few ways to determine what your BIOS is:
SIW software (free download) SIW | Download
fill find it for you! Along with all other information concerning your motherboard and installed hardware/software.

Find out the BIOS version and date
Go to the computer's manufacturer web site and see if they have a BIOS update for your model of computer/motherboard and down load it...save to desktop.

Read the BIOS information that is located for the update and if they have an updater then I highly recommend that you download it and use it to flash(update) the BIOS.

If there is no newer BIOS update from the manufaturer then you can try the actual BIOS manufacturer and see if they have one. SIW will link you to them for the BIOS that you have.

Follow the directions exactly as they are given for FLASHING the BIOS...
*****NOTE.....this is a last choice and all risks are held by you and cannot be held against anyone else!!!!

Good Luck!!

Dvd Rom changing solved the problem so what should i do with the old one? besides throwing it troughout the window

Great work.
The drive needs to be recycled. You can look for an electronics recycler in your area or your local city houhehold goods recycling center. Ethier of those should take it for free.
Go to Google, put in Electronics Recycling + nearest city and you should come up with several options.

GO GREEN

It is a good habbit to get into!