The short answer is that you don't need one and, most likely, can't have one.
First of all, you should have specified the exact phone model. Very few Sony Ericsson models run the Symbian operating system - most of them use proprietary operating systems, for which neither viruses, nor anti-virus programs exist (or are possible).
But even those phones that run the Symbian OS use the UIQ flavor of it. Mobile phone viruses threaten seriously only the phones running the Symbian S60 R2 operating system. Only 3 viruses exist for Symbian UIQ and none of them has any chances of surviving in the wild. So, buying an anti-virus program for a Sony Ericsson phone, even in the rare cases when it is possible, is a waste of money.
Sonyericcsion mobile phone anti virus?norton antivirus
Dont think there is 1...why would you need 1?
Sonyericcsion mobile phone anti virus?security
"what is your question?"