SOunds like on the early symbian phones like Nokia 6600 someone cracked a game but put a "virus" in it to keep texting a premium rate no.
A k750 doesnt have an operating system as such though
I would go with a factory reset via the settings menu which will delete any data on it. (Inc No.s / Photo's etc so anything important back up) could also do the update of software via sonyericsson.com
If you get stuck shout as i'm fairly good with SE phones, actually flashing a W800 now from scratch as it had the flashing white screen.
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