the firmware is corrupted. flash the phone (sometimes called debranding) I use www.totalgsm.org
not the cheapest but good support. follow the instructions to the letter takes about 10mins to do. see my other posts for more detail. will cost 10euros lots cheaper than getting it done by a shop & that is what they'll do anyway. use generic firmware rather than operator branded firmware. it will run better, & none of those nasty logos. furthermore any function that has been disabled by your operator will be restored (like switching off 3G or **** that you cant erase)after you've done this send MMS & Internet settings using SE website to complete the process. this can be done for free on www.topsony.com
BUT you have to put at least 20 posts on their forum so if time is an issue, dont bother. total gsm is an easier way if you've never done fone flashing before.