Montesinos fled Peru as the scandal unfolded, but was later arrested in Venezuela.
The former intelligence chief, who is also serving a jail term, himself faces charges of corruption, drug trafficking, and selling arms to left-wing guerrillas in Colombia.
Fujimori has already incurred a number of other court rulings.
In 2007, he was sentenced to six years of imprisonment for directing aides to steal incriminating documents from the house of Montesinos's wife.
He was also sentenced to 25 years in prison in April after he was found guilty of permitting the operations of an army death squad that killed 25 civilians in 1991 and 1992.
The corruption case comes weeks after an opinion poll showed that Keiko Fujimori, the former president's daughter, is the leading candidate in the run-up to presidential elections set to take place in 2011.
She has said that she would pardon her father if elected, although it is not clear if she would have the constitutional authority to do so.