Computer screwed up

My family computer is messed up. I've gotten fed up and
need an answer. When it starts you can choose your login
name on XP but after that it wont let you do anything
else. I believe one of my idiot brothers or step mom
opened an e-mail or downloaded a song or many songs that
screwed up the whole comeputer what can i do to help it
without spending money to take it somewhere to get it
fixed?

Computer screwed up