Delta Blue Squadron

The two rivals have been locked in a drawn-out battle to stand against Republican John McCain in November’s presidential vote.

Unassailable lead?

Estimates suggested brisk turnout in both primaries on Tuesday.

Indiana is home to large numbers of blue-collar workers, a group which has backed to Mrs Clinton in previous contests.

In North Carolina, Mr Obama is expected to benefit from the votes of African-Americans, who make up more than a third of the electorate there.

Mrs Clinton has cut Mr Obama’s lead in North Carolina to single digits in most polls over recent weeks and the two have been close in Indiana, where she has had the edge.

Before voting began, Mr Obama told reporters in Greenwood, Indiana: “I think we’ve campaigned hard. I think it’s going to be close. I’m seeing a lot of enthusiasm.”

Mrs Clinton told reporters in Indianapolis: “Every race is filled with the unexpected. It’s like life. You never know what’s going to happen.”

Mr Obama, an Illinois senator, has an almost unassailable lead in delegates, the party officials who will choose the nominee at the Democratic convention in August.

Altogether, 187 delegates are at stake in Indiana and North Carolina, nearly half the remaining total. Six state contests are left.

Mr Obama was leading the race in delegates by 1,745 to 1,608, according to an Associated Press count on Tuesday. But neither can win enough delegates to clinch the race before voting ends on 3 June. This means that nearly 800 super-delegates - senior party members - will have the final say.

Clinton ‘re-energised’

The two Democrats, courting voters suffering from an ailing economy, have spent recent days sparring over Mrs Clinton’s proposal to suspend the federal petrol tax for the summer.

Mr Obama and many economists called the plan a political gimmick that would save little money for most families.

But Mrs Clinton launched an advertisement in both states questioning her rival’s stance. She said the plan would be financed by a windfall tax on oil firms.

Analysts say Mrs Clinton saved her candidacy and has been re-energised by her win in Pennsylvania two weeks ago.

Mr Obama has struggled through a rocky campaign stretch, dogged by controversy over his gaffe that small-town residents were “bitter” and fiery criticisms of America by his former pastor, Rev Jeremiah Wright.

Mr Obama received a publicity boost on Monday with an endorsement from Hollywood star Tom Hanks and narrowly beat Mrs Clinton the day before in a caucus at the tiny Pacific territory of Guam.

Because of the way the Democratic Party distributes the vote, each candidate won two delegates.

The remaining Democratic primaries will take place over the next month in West Virginia, Oregon, Kentucky, Puerto Rico, Montana and South Dakota.

Signs of trouble

The FBI has been trying to tackle networks of zombies for some time as part of an initiative it has dubbed Operation Bot Roast.

This operation recently passed a significant milestone as it racked up more than one million individually identifiable computers known to be part of one bot net or another.

The law enforcement organisation said that part of the operation involved notifying people who owned PCs it knew were part of zombie or bot networks. In this way it said it expected to find more evidence of how they are being used by criminals.

“The majority of victims are not even aware that their computer has been compromised or their personal information exploited,” said James Finch, assistant director of the FBI’s Cyber Division.

Many people fall victim by opening an attachment on an e-mail message containing a virus or by visiting a booby-trapped webpage.

Many hi-tech criminals are now trying to subvert innocent webpages to act as proxies for their malicious programs.

Those in charge of botnets, called botherders, can have tens of thousands of machines under their control.

Operation Bot Roast has resulted in the arrest of three people known to have used bot nets for criminal ends.

One of those arrested, Robert Alan Soloway, could face 65 years in jail if found guilty of all the crimes with which he has been charged.

In a statement about Operation Bot Roast the FBI urged PC users to practice good computer security which includes using regularly updated anti-virus software and installing a firewall.

For those without basic protections anti-virus companies such as F Secure, Trend Micro, Kaspersky Labs and many others offer online scanning services that can help spot infections.

The organisation said it was difficult for people to know if their machine was part of a botnet. However it said telltale signs could be if the machine ran slowly, had an e-mail outbox full of mail a user did not send or they get e-mail saying they are sending spam.