Try using
window.open('{!URLFOR($Action.Contact.NewContact, Contact.Id )}');
Update : try this : click on inspect element on the lookup button and you can find Id of the lookup as con4_lkwgt (I would assume this to be same since its a standard field) and click on the account name field and get the ID as con4
/003/e?con4={!Account.Name}&con4_lkwgt={!Account.Id}&&retURL=/{!Account.Id}
Refer this blog for more insight on how to override the buttons http://raydehler.com/cloud/clod/salesforce-url-hacking-to-prepopulate-fields-on-a-standard-page-layout.html