Failed file transfer

I have a laptop linked by WiFi router to my two cable networked
desktops. All computers run XP SP2, and everything on the network
functions fine - cross network printing, web acccess, e-mail etc. -
except for file transfers. If I try to copy/paste or drag/drop a file
from the server to the laptop, it works just fine - but the first time
only! After that further transfers repeatedly fail. On trying to use
cut/copy and paste the "paste" option disappears from the right click
menu. Obviously all necessary permissions and sharing rights are in
place, or the first transfer would not work. Any ideas why it should
suddenly stop functioning?

Regards,

Bob