The whole point of user accounts and privileges is that you can't just bypass them and do what you want. Once Windows has booted, barring any security holes in software running as a privileged account, you can't just "bypass" security.
However, depending on what the virus has done, you may be able to use the Offline NT Password & Registry Editor to enable and reset the password on the Administrator account, then use that account to log in and hopefully remove the virus.
If the virus somehow still locks you out while using the Administrator account, then you can try booting into safe mode (press F8 during startup) to see if that might help. If not, you may need to format the hard drive to really eliminate the virus. In fact, I recommend you plan on going that route anyway sometime soon since it's the best way to be sure the virus is completely gone.