How to transfer pidgin settings from one computer to another

C:\Documents and Settings\<username>\Application Data\.purple\

The entire directory can be brought over, or if you just want specific pieces look at the individual files, they're pretty self explanatory (accounts.xml, prefs.xml, etc). Put them on the Windows 7 machine in

C:\Users\<username>\AppData\Roaming\.purple

You can set a PURPLEHOME environment variable to tell Pidgin where it should store (or look for) its .purple folder. I'm not sure if it's created by default, but it certainly helps in managing the directory afterward. Basically you just need to copy that .purple folder over, which will probably be in %appdata%.

More info here: http://www.lib.unc.edu/reference/eref/pidgin/env_var.html