Phone Virus? Sony Ericsson w850i.?

Hi. ill try to be as specific as possible. i downloaded a ton of themes and ringtones for my sony ericsson, and it was working fine until later on in the evening. i downloaded them to my pc, and then via usb cable transferred them to my phone. i do not have internet on my phone. after that, my phone started being weird, and then i was playing with settings and everything got deleted off my phone- pictures, ringtones, themes. i traced my steps and it didnt look like i did that myself. now, when i turn on my phone, it says "insert a sim card or view demo of this phone" even though i already have a sim card inserted. and when i Do get it to turn on properly (after about 4 tries) the same thing happens only a little after it turns on. could this be a virus? im really confused because i know that i dont have a symbian (sp) phone. again, it was working fine when i first got it. if it is, what do i do to get rid of it? can i take it to a phone store? thank you so much.