Hardware Extra ram now PC wont start

It works normally if you take the memory out again?

Maybe it only accepts 2GB sticks?

The PC has 2 slots and accepts up to 8gb. I put the an extra 4gb of ddr3 in and it wont boot so 2gb in one slot 4gb in the other. The bios sees the 6gb but wont boot with the whole 6gb in or with just the 4gb. The PC just goes to repair and if I select that I just get a black screen for how ever long I leave it, if I put the original 2gb it will boot but I get a DOS message everytime I boot saying RAM changed and select F1 or F2.

How can I check if the ram is faulty ? It was bought new.

Download an iso with a memory checker option such as a Linux Live CD, it's a boot option. Also worth checking if the PC will boot another OS.

Just to correct you, it's not a Dos message it's a bios message. And this message is normal for any changes to the ram.
Have you tried just putting the new 4GB ram without the 2GB ram (just to test the memory)?
Also be aware that windows comes in two flavours, 32Bit and 64bit. If you're going to be using all the 6GB of ram you have then you would need a 64bit windows version. as the 32bit version only addresses ram up to 4GB.

Can you provide the part/model numbers of both of the RAM sticks, and the motherboard, please?

Its ok, turns out the ram was faulty.I eventually ran some test on the ram and it just about failed them all.
I bought another 4gb stick and the PC's working fine. Thanks