Open Question: Privacy on computer????????????????????????????????????

I have windows XP...and I made user accounts...so here's a long story short...my roommate went and told a few people some of the stuff that I was talking to my BOYFRIEND about...completely pissed me off and I didn't know how she got her information. So anyhow...I had a mutual friend sit with her and ask her to show her the conversation between my BF and myself and...I come to realise that even tho we have seperate accounts on the computer...she can still access my message history...I dont' want to delete anything at all....but what I do want to do is make it so that it's private that way she can't access them at all...can someone please help me?? Thanks a bundle