mac - pc file sharing = corrupt files

Ok this has me stumped.
Last night I tried to copy a video-cd format mpg file to my win xp pro machine by dragging & dropping in the finder. When I tried to open it on the pc it said invalid format, so I went back to the mac and zipped the mpg and copied again. When trying to run unzip on the pc it reports a crc error in the zipfile, but the original is ok. I then used the terminal and cp command to copy the file and it is fine.
I have copied files from the pc to the mac with the finder hundreds of times without this problem. It only shows up when copying from mac to pc.

Any thoughts? I have 2 120G hard drives setup as a single striped raid drive on the power mac, could that be problematic?

My setup:
MDD 1G powermac
80 HFS+ drive
2 120G HFS+ striped raid drives

Athlon 1800+
win xp pro
120G NTFS drive (20G and 100G partitions)

Nexland Pro800 turbo router