I had 256MB RAM DDR PC2700 in my PC and I installed a 1GB stick. I had run a memory needs check from Crucial.com and it told me what I needed and I also checked the actual label stuck on the RAM in the PC.
The connections on the RAM look ok - not damaged - and fitted into the slots without a problem.
Now when I power up the PC the button goes green and the fan on the motherboard kicks in for about 5 seconds and then it stops and the light goes off and a few seconds later the button goes green again and the fan starts again. This keeps happening until I hold the power button in on the PC to power it off. I checked all the wires and connections and can't see find anything loose. It doesn't beep at all.

Any suggestions please?

2 questions .dose it work with old stick in the slot ,second ? ,did you buy a new stick of ram

Does the system make any beep noises when turned on? These can often explain where a problem lies. They often sound like morse code.

If you are keeping the 256Mb stick in the machine aswell, I'd try putting the 1Gb stick in slot 1 and the 256Mb stick in slot 2.

is it an older machine? im guessing that it is with only 256MB.

you need to check if your mother board is capable of handling that much ram. some Mo Bo's will only handle so much ram.

to add to what he said, if you are keeping the old stick in make sure they are not in the same colored slot (put them side by side, don't stagger them) the reason for this is that you are not using Dual Channel Ram and it could mess up your computer if you try.

even if it is new RAM, i wouldn't be surprised if it was bad. RAM is one of the most feeble things in a computer.