Hepatitis C is a liver disease that is caused by the hepatitis C virus (HCV). It can be successfully treated with 6 to 12 months of medication in both HIV infected and HIV uninfected people. Among HIV infected people, HCV infection is a common co-morbidity, and is more serious when it occurs in this population than others because it leads to liver damage more quickly. HIV is known to cause neurological deficits, and studies suggest that HCV may do so, as well. Knowledge about how to treat these deficits, however, is limited. More information about the nature of the neurological problems and their causes is needed to develop effective treatments. This study will determine the effects that HIV and HCV have on thinking abilities, such as memory, attention, and problem-solving, and whether the viruses affect brain chemistry.