Are you sure you're trying to log into the account labeled "Administrator" (with the chess user icon)?
If you are sure, then that means the computer was pretty well secured by whoever sold it to you. In order to gain access you're going to have to break the password on your account using something like a live Ophcrack cd. If you're not very experienced with computer repair and terms, I'd reccomend going to a local tech shop (not a retail one like Best Buy's geek squad, they won't do what you want) with your parents (if you're under 17/look very young) and ask them to recover the password for your account. Chances are the guy will know a bit about password breaking.
If you are technically inclined, then you can give it a try, it's unlikely that you could damage your computer, however you may find it difficult to figure out if you're just using computers in their simplest way.
To understand the method usually used to "break" passwords, you have to know the concept behind them. In most modern operating systems, passwords are stored as something called a "hash". This hash is the result of an algorithm applied to your actual password (i.e the password: omg5572$ when hashed using the NTLM algorithm (windows authentication algorithm), results in "C6DEA3660D1F5EE25AB002F614D342E0" without the quotes. ). This hashed version is stored in a file. Now, the thing that simply prevents someone from going into said file, copying the hash and pasting it into the login prompt is that the login prompt hashes whatever is inputted into it, and then trys to match the result with the stored hash. Because of this, if you entered an accounts hashed password, the login manager would then hash the hash you inputted (inception :O) resulting in a totally different hash being compared with the accounts stored hash.
So one has to determine what that hash's password actually is. In order to do this, one uses a program to generate millions of hashed versions of random word or letter(/number/special character) combinations, until a match is found. Once a generated hash matches the account hash, the program knows that the password used to generate that particular hash is the account's password.
Depending on the strength of the password (how long it is, how complicated and random it is), breaking it may take minutes, or hundreds of years on a normal computer. The password "lolhai" could be broken within half an hour on a good computer, but the password "lgkhetgs332&&%332_fhf2FDF2er2823" could take years, or longer to break, due to it's complexity.
Here's a tutorial you can try to follow if you'd like.
