From Carrie Higbie, Ask the Expert , SearchNetworking.com
I would also try moving to another switch port. I would also check to see that the switch port is set to full duplex. You can call a cabling installer and have them test the cable for you that should rule out the horizontal part of the cable and both patch cords. If the cabling tests and passes (which I believe it won’t) then it will be in your electronics. But more than likely the cable is faulty or the NIC card or switch port is faulty. In the past we used to have to set switch ports to 10Mb half duplex for some applications. If you are running half duplex you will have collisions.

From BuddyFarr
if you take the workstation to another connection on the same network does it still do this? if not I would suspect the patch cable on either end or the horizontal cabling, (cable in wall between wall jack and wiring closet). If it still happens it could be the NIC in the workstation, the drivers for the NIC or badly written software that you are using. What software are you using when this happens? Depending on how much data you are pushing across the wire you can still get some collisions. Does it happen all the time? Is the collision light on the switch lit constantly?

From jmkelly:
Collisions aren’t the end of the world. Rich Seifert, one of the developers of the original Ethernet protocol, used to say he wished they’d never used that word, but called them something like “medium access arbitration events.” Even in a perfectly functional Ethernet LAN, collisions will happen; they happen whenever two hosts try to talk at once, which is going to happen with a non-deterministic protocol. (If you really hate collisions, dump Ethernet and run Token Ring!) Following a collision, each host backs off and tries again later, thus keeping congestion *on the edges of the network*, which is the whole purpose of the “collision” part of the protocol. The question is not whether you’re getting collisions; it’s whether you’re getting all the bandwidth the equipment
is supposed to provide?
BTW, cable faults can’t cause collisions. They might cause dropped frames and retransmissions,
which might lead to collisions, but then lots of things can cause drops and retransmissions.