Computer won't boot

[Archaic`]

Recently got to take home an old PIII from work that they were going to throw away, and knowing my father had a computer of the same generation, offered him the RAM from in it as an upgrade, intending to replace the 128MB stick in there with the 256MB from this, bringing him up to 512MB total. Put it in, turned it on, computer wouldn't boot. Looked at the RAM, saw it was a double sider while his existing 256MB stick was only single sided, and put the old single sided 128MB RAM back in. Now....it still won't boot, while the one I brought back from work is working fine with its original config. What am I doing wrong? Is it just that I pressed the RAM in too hard, or not hard enough, or have I likely screwed it in other ways?