It'll be your only way to see if it's a software problem or not. Either your power button becomes your home/back button (ie reboot), or you factory reset and see. If cannot then you might want to try rooting it, since warranty already ended. Unless you want to continue using it in its current state...

Or bring it to a repair shop and see what they say. This is the first time I head the capacitive buttons not working on an unrooted phone. Only time I heard it is in the early days of flashing CyanogenMod 7 where the keypad mappings for the Desire Z weren't included in the Vision package, only the G2 one.

doing a quick search in Lelong for Desire Z screen returns one seller with the digitizer selling at 179 and another one providing the repair service. you can try contacting them

http://list.lelong.com.my/Auc/List/List.as...+screen&x=0&y=0