Have a look at this and select the network its locked to: How to unlock Sony Ericsson XPERIA X10 Mini Pro | giffgaff unlockapedia
I dont think you will find a free way for that phone, even many older Nokias can not be done for free at home anymore. Its usually cheaper to pay the network to unlock it, or take it to a phone shop that can. Its not usually as easy as just connecting the data cable and running some program on your pc anymore.
If you have had the phone longer than 12 months your network provider will provide a free unlock code as part of there service