Copying file restarts recipient PC

I have a LAN with a x4 router and XP Home in all the PC’s.

When I try to copy a file from PC A into a shared folder in PC B, B is immediately turned off and restarted, like when pushing the reset button (it happens the very moment I hit Ctrl+C).

This is something I do quite often and never had this problem before.

If I do the same when sitting at B (i.e., I GET the file with B instead of SENDING it with A) it works fine.

Any ideas as to the reason and/or the solution?

Thanks a lot.