Are you sure your Acer 5100 is a desktop? It seems to be a laptop according to Google.
Faster RAM wouldn't prevent it from booting--it would just clock down and run slower at the speeds his motherboard is capable of.
I've accidentally not put my ram all the way in lots of times and the computer won't boot without ram connected. Triple check that the ram is the same type (DDR2, not DDR3) and that it is plugged in correctly.
If it boots with your old RAM, but not your new RAM, then return the RAM.