Hi,
On your pix make sure you are on config mode i.e.
pix(config)# interface ethernet inside 100full
...if you want the pix interface outside to be running at 100 full duplex than do
pix(config)#interface ethernet outside 100full
*Make sure you write to memory i.e. wr m after you have changed the ethernet speed* also a reload of your pix will be good.
Hope this helps.
Jay.