Problem in reformatting computer. Forgotten password

Our computer is 4 yrs. used, and it's started to hanged up always. When I'm going to opened the computer, it's very too slow in the startup and the icon doesn't show for 1 – 2 hours. I tried to reformat my computer but the problem is, it's asking for a password. My uncle is the last who reformat our computer, I didn't know that he did set a password. I asked him the password but unfortunately he forgot it.
Guys, is there anyway how to reformat our computer? Even we can't remember the password? I really need to use the computer, our class is started last week. Please need some suggestions, thanks.
