Files deleted when copied over network

My Mac's hooked up to a Windows machine. I can mount the Windows HD on my desktop, and copy files onto it. However, sometimes when I copy files from Mac to Windows, I see the file copy, it takes a while if it's a big file, then it just appears to delete itself off the Windows machine. Try it again and it usually works, although sometimes I need to try 5 or 6 times before the file will stay there.

This doesn't happen all the time, sometimes a file will copy perfectly, other times it takes several attempts to get the file to stay there. It seems to happen most with .exe files but I have seen it happen with other file types too. It also seems to happen most in the root directory - sometimes I end up making a folder and copying into that, which usually works.

I never have this problem when going from Windows to Mac, just Mac to Windows. My Mac's running 10.2.6 and I don't think I've installed any "special" networking software. The PC's got Windows 2000. I've got a suspicion that my anti-virus may be the culprit, it's Trend PC-cillin 2002.

Any ideas?

I'll need to do some testing with the AV turned off, it's hard to test because it's intermittent. I tried copying a pdf across today and that disappeared too. Second attempt worked.