Hang on if you bought it as a pre built system from a shop or Pc world for example, you should of took it back there if it was under warranty. It would of been down to them to fix the problem as it was only a week old, even if pre built by a computer shop.
I would take it back if you can as it is likely they put a cheap PSU in it causing the problem in the first place, Plus it may of also damaged other parts of the computer when the Psu blew. Why pay more money when clearly you should not. Unless you bought it off a friend that is. Put the old Psu back in and take it back to the shop.