ok! it's either a problem with the motherboard or the upper slot (or both). Both dimms work fine in my sister's computer.
When I tried her 4gb dimm on my computer in the upper slot, it worked fine (i tested it for 72 hours) I was happy thinking it was a problem with my dimms. Then I tried my 4gb samsung dimm on the upper slot again and it was working fine, which was really strange...
(I ran the test for like 2 days).
Then I decided to try ( i don't know why) my sister's dimm again, and after some seconds of having started memtest a lot of errors appeared and computer restarted, then i tried with mine and the same, after that i ran memtest lots of times (alternating my 4gb dimm and my sisters) and it always showed errors within 20 minutes of having started the test (sometimes after just 30 seconds).
I just don't understand why both of them worked fine for days and suddenly this.
Here you can watch 2 videos, check the weird "lines" and characters the screen starts showing (sorry for my poor english) in the 2nd one when the screen goes black its because the computer turned off by itself and then I couldn't turn it on, I had to wait for some minutes...
This is making me so stressed, thank you guys for helping me.
http://youtu.be/8_tMvzW1Wgc (television was on haha, i am not saying anything)
http://youtu.be/uNXD9wHf-fo
http://youtu.be/JUEjv1nuuEg memtest confeti lol