there isn't one. depends on what your system (OS) is, what you want to copy, etc.
assuming you are running windows, one way is enable sharing (probably control panel -> network settings), share your entire "C:" drive from the source (old) machine, "map" (tools->map network drive, in windows explorer) it (\\old-machine-name\C$) on the new machine (as another drive, say F:). you can now browse through the drive and copy files you want.
in reality, you probably want your user data, in specific application data. windows tends to distribute user data over quite a few places, so this is a bit of a pain. if the source host is a newer windows system, you can look in the "C:\Documents and Settings" directory, and perhaps copy over user directories from there to the new host (be warned: i have never done this stuff and i am not sure what the effects of over-writing existing files may be).
applications themselves will mostly need to be re-installed. OS customizations, settings, add-ons, patches, etc are probably also better off being manually repeated on the target host. in particular, copying over the windows registry is probably dangerous.
otoh, if you want your new computer to look exactly like your old, just pop out the hard disk from the old and plonk it into the new (you may need to set master/slave jumper settings to have your system boot off the old disk).
i hope this sketchy advice is of some use. thats about all i know of windows...
--ravi