You will find that you must use Google account to sign in Orkut. There is a letter from Orkut, it says:
Dear orkut members,
We are now transitioning to use Google Accounts to sign into orkut.com.
Sign in with your orkut username and password.You will be asked to link this with a Google Account. You can also sign in with an existing Google Account if you have one. For example, a Gmail address is a Google Account and will work.
With a Google Account you will be able to use other Google services like the Google Personalized Homepage, Google Alerts, Google Groups, Froogle Shopping list, and many future Google services. Google Accounts are also better protected against fraud, impersonation and abuse.
– the orkut team