And how can I download memtest if i can't do anything????

Did you try removing the memory and or swap memory modules and trying one at a time if you have two? And use a can of duster air to blow out the memory slots.

Also try downloading memtest and test your memory.

Start with the Dell diagnostics - F12 at powerup.  Run a test and record which DIMM (A or B) produces the error.

Then swap the two modules.  Re-test.  If the error follows the module, replace that memory modules.

If the error sticks with the same socket (and the other module), replace the system board.

But the problem is that I CAN'T start the Dell diagnostics, I press  F12 key but it doesn't work,

Take out both modules and try each one at a time -- if one module is bad, it may be preventing the system from starting.

What "told you" that - did the system boot into WIndows?  If these are Windows errors, they may indicate a corrupt Windows install.  What happens when you go into the Dell diagnostics (F12)?

Ok now, I tried to do what you told me to do, but while doing that it said that the memory couldn't be read, do i need a technician for this???Am I really screwed???