go here http://www.hirensbootcd.org/download/ download and burn the software to a cd using another computer. boot from cd and select miniXP you'll be able to use password recovery tools.
or during boot press F8 and enter safe mode, and try to go in through the hidden administrator account there. There may not be one however if it still has vista or win7 on it, there is one on Win XP that a lot of people never secured with a password.
We can't provide assistance with by-passing/cracking any password protection, as we can't verify ownership of the system.
You will need to contact the vendor or the manufacturer with proof of ownership/purchase and they will be able to help.
Sounds like a crappy Pawnshop. Depending on the pawnshop, it might be stolen.
When i hocked my computer, they made me boot it up and checked everything. However, I did a loan on it, but either way they should have checked this, I would return it, and next time check the computer before you buy it.
I wouldn't by a guitar from a pawnshop unless I tried it, neither would i with a computer.