Can I change my SIM card on a Sony Ericsson W810i?

I bought a Sony Ericsson W810i from Argos with the O2 package, but I'd like to change my SIM card to Virgin. Can I do this, or is the phone only registered for O2?

No, it's illegal

Yes,You will be able to swap your sim card.
For changing your sim card,Make sure your phone is free from any network lock.If it's locked by particular network,you will need to unlock your phone.For unlocking visit MobileUnlockSolutions.com Here you can unlock your phone at low cost.