Outlook stores them in the PST file, open the PST file in question
through Outlook, File, Data file management Add. and then drag the
content of the Contacts folder from old file into the new.
It depends on where you are storing them and which type of contacts you are
referring to.
Assuming all your contacts are kept in Outlook itself (after all, this is an
Outlook newsgroup ;-)), you can migrate your data by copying over your
pst-file from your XP computer to your Windows 7 computer. Make sure you do
not overwrite any existing pst-files in this process.
After you have configured Outlook on the new computer, reconnect your
pst-file via;
File-> Open-> Outlook Data File...