Well, There are two things you can do. You can try and contact the last owner of this laptop and try and get the password from them. Although, If you don't mind losing any data you can system restore your PC to how it was and then you will be able to make a new account on this laptop.
To system restore follow the instructions CAREFULLY below:
1.Turn on the laptop and when it boots it will appear with an Acer logo, When it says this mash F2 or F8 and a screen should appear. This is called the BIOS.
2.Now press the arrow keys until you reach a tab called "Boot" or something like that. When you have done that it should come up with about a list of 8 things. This is the booter order the PC will follow to boot the PC up successfully...