If you have a history of violating the law and are charged with a new crime, then you may be concerned about your past hindering your ability to defend yourself in this case. You're partially right in your concern; there is a higher likelihood that your past could be brought up.
Your past criminal records will be available to the police and prosecutors, even if you've had your record expunged. These past convictions may affect what kind of plea bargains you'll be offered and if you will have certain kinds of charges filed against you. For instance, if you have a single drunk driving conviction from several years ago, you'll likely face a harsher penalty for a second offense than if you have nothing on your record.
Past criminal behavior doesn't automatically mean that you committed a crime this time, and the courts are aware of that. Your history of a DUI, for instance, is not enough to show that you would be a drunk driver a second time, even though it might make sense. Instead, there needs to be full evidence that you violated the law and that you did drive drunk a second time.
Your past in the criminal justice system can make a difference in how you're treated, but every case is its own challenge. Your attorney will talk to you about your options and what you can do to help yourself fight off any new charges against you. Our site has more information on how you can protect yourself if you're accused of a crime and have a criminal past.