Forgotten passward under administrator acc.

Hello, i bought this computer from a neighbor that was moving mostly for my son for that price as long as it had internet..... the problem was my son was getting to know his way with the files so i had to create an account user for him..... he was still able to figure things out for and 8 yr. old so then i start to set a password for my main account and eventually i was able to lock my files from him and he could not download on his account.... but one day he figured out my account and was going into the main and so i quickly changed it while in the middle of cleaning and just plum forgot what it was...... so is there anyone that can help..... when i bought the computer the guy had the cds packed away so i have nothing.... can anyone help me with a forgotten password.....if i were to clean out the systsem would i be then be able to start from scratch... my friend has a windows xp .... can i use his to re do my computer.....