1. A method for treating cancer by in vivo gene therapy comprising the step of administering to a subject a therapeutically effective amount of a viral vector, characterized in that said vector comprises a gene that encodes interferon-beta, and wherein said vector is selected from the group consisting of an adenoviral vector, a lentiviral vector, an Epstein Barr viral vector, a baculoviral vector and a herpes simplex viral vector. 2. The method according to claim 1, characterized in that the vector is selected from the group consisting of an adenoviral vector, a baculoviral vector and a lentiviral vector. 3. The method according to claim 1, characterized in that the vector is an adenoviral vector. 4. The method according to claim 3, characterized in that the adenoviral vector has a deletion in its E3 gene. 5. The method according to claim 3 or 4, characterized in that the adenoviral vector has a deletion in its E1 gene. 6. The method according to claim 1, characterized in that the vector is administered by a route selected from the group consisting of direct injection into a tumor, topical administration, intraocular administration, parenteral administration, intranasal administration, intratracheal administration, intrabronchial administration and subcutaneous administration. 7. The method according to claim 6, characterized in that said route is a parenteral administration route selected from the group consisting of intravenous administration, intramuscular administration and intraperitoneal administration. 8. The method according to claim 6, characterized in that the vector is administered by direct injection into a tumor. 9. The method according to claim 1, characterized in that the tumor is selected from a group consisting of malignant glioma, melanoma, hemangioma, leukemia, lymphoma, myeloma, colorectal cancer, non-small cell carcinoma, breast cancer and ovarian cancer. 10. The method according to claim 9, characterized in that the tumor is malignant glioma. 11. A method for treating cancer by in vivo gene therapy comprising the step of administering to a subject a therapeutically effective amount of a replication-defective vector, characterized in that said vector comprises a gene that encodes interferon-beta, and wherein said viral vector is selected from the group consisting of a adenoviral vector, a lentiviral vector, a baculoviral vector, an Epstein Barr viral vector and a herpes simplex viral vector. 12. The method according to claim 11, characterized in that the vector is selected from the group consisting of an adenoviral vector, a baculoviral vector and a lentiviral vector. 13. The method according to claim 12, characterized in that the vector is an adenoviral vector. 14. The method according to claim 13, characterized in that the vector is deleted in one or more genome regions selected from the group consisting of the E1, E2A and E4 regions of the adenoviral genome. 15. The method according to claim 13 or 14, characterized in that the vector has a deletion in its E3 gene. 16. The method according to claim 14, characterized in that the vector is deficient in its E1 gene. 17. The method according to claim 16, characterized in that the vector additionally has a deletion in its E3 gene. 18. The method according to claim 11, characterized in that the vector is administered by a route selected from the group consisting of direct injection into a tumor, topical administration, intraocular administration, parental administration, intranasal administration, intratracheal administration, intrabronchial administration and subcutaneous administration. 19. The method according to claim 18, characterized in that said route is a parenteral administration route selected from the group consisting of intravenous administration, intramuscular administration and intraperitoneal administration. 20. The method according to claim 18, characterized in that the vector is administered by direct injection into a tumor. 21. The method according to claim 11, characterized in that the tumor is selected from a group consisting of malignant glioma, melanoma, hemangioma, leukemia, lymphoma, myeloma, colorectal cancer, non-small cell carcinoma, breast cancer and ovarian cancer. 22. The method according to claim 21, characterized in that the tumor is malignant glioma.