How do i change my mian picture on my computer?

like when i logg onto my computer and it says ur name in a lttile box and u press on it and put your pw in well dats wat i do and then i pressed start to c if its thier but it wasnt. (LESS CONFUSING VERSION): when i press start and look at the top it doesnt hAVe that litttle box instead it says windows xp proffessinal in blue on the side SUM 1 PLSZ HELP MEHH|||Ask it in a less stupid way (ie grammar, punctuation, spelling, correct capitalization) and I%26#039;ll tell you. These are things you learned in the third grade. Use them.