Tuesday is the day computer hackers have had circled on their calendars for a long time.

Microsoft Corp. on Tuesday will stop issuing security updates for its popular but old XP Windows operating system, which means millions of personal and business computers will be susceptible to hacking in a multitude of new ways.

“The bad guys know about the loopholes and are waiting until April 8 to exploit them,” said Dave Greenbaum, owner of Lawrence-based Doctor Dave Computer Repair. “It is dangerous for a home user to be using XP after April 8. It is incredibly dangerous and borderline malpractice for a business to be using it.”

Since Microsoft no longer will be providing security fixes for its XP operating system, that means hackers will have new ways to access email accounts, personal information stored on computers, and new doorways to insert viruses.

Computer technicians in Lawrence said they believe many area residents are going to be caught unaware of the change. Microsoft began alerting users more than two years ago about the discontinuation of the XP service, but the issue hasn’t resonated with many users.

“There are a lot of people who don’t know what OS means or what XP is,” said Amin Emami, general manager of Lawrence’s UNI Computers. “I would never tell people this is something they don’t need to worry about. This is something they need to worry about.”

Chances are, there are lots of XP users in Lawrence. At the beginning of the month, PC World reported that XP was powering about 27 percent of all the personal computers operating in the world.

Greenbaum said the number of calls his business is receiving about upgrading computers is increasing, especially from small businesses. He said businesses ranging from doctor’s offices to retailers have been calling with concerns they may no longer be HIPPA complaint or their online credit card systems may no longer meet standards.

Issues like those are what make the XP problem more complicated. Greenbaum said an individual may have updated his computer but still ends up having problems because he interacts or does business with another user who has not updated.

“You really have a duty as a responsible computer citizen to update,” Greenbaum said.

As for the process of updating to a new operating system, both Greenbaum and Emami said it probably is going to be more cost-effective to buy a more modern computer than to install a new operating system on an old machine. Generally, Greenbaum said any computer with an XP system is going to be at least four years old. Microsoft stopped selling Windows XP in 2008, but some manufacturers were allowed to sell machines with the XP software into 2010, Greenbaum said.

Anyone uncertain whether they are running XP on their computer can have it checked at various websites, including amirunningxp.com

Emami, who sells personal computers at UNI, said a computer can be replaced for $200 and up.