Where do collisions come from?

I had a happy little network of six users all running windows 95/98 peer to peer. The network was reasonably fast all 100BaseT ethernet and ICP/IP with fixed IP addreses. A certain large file transfer would take about six seconds. Since we neededmore storage I added a Network Attached Storage. This is a rack mounted unit with 60 GB of hard drives and Linux operating system. The same file transfer to the new server now takes over two minutes. The collision light is on almost the whole time. Every thing is the same why do the collisions suddenly appear?