The spread of the MyDoom worms slowed markedly Thursday, as computer users learned how to protect their machines from infection.

No hard numbers were available, but Internet security firms said e-mail traffic was normal, in contrast with the surge of messages sent by the MyDoom worms beginning on Monday.

The worms — a kind of aggressive computer virus — are expected to remain quiescent at least until Sunday. That is when MyDoom and its cousin, MyDoom.B, are set to swamp Microsoft Corp. and SCO Group Web sites in an effort to crash them.

“Typically you will see a slowdown for a number of reasons,” said Brian Czarny, marketing director of MessageLabs. “As people get more educated, they stop opening the attachments.”

Both MyDooms spread via e-mail bearing an attached file. The official-looking e-mail — some bearing words like “system administrator” or “server report” — contains an icon indicating an attachment. Anyone clicking on the icon will activate the worm.

The MyDoom worms garnered much media attention this week, and that is warning people about suspicious e-mail, security experts said.

Infectious e-mail is melding with spam to create yet another annoyance for people dependent on the Internet.

“The virus issue is converging rapidly with the spam issue,” said Chris Belthoff, senior security analysis with Sophos Inc., a provider of computer security systems.

Spam is the unwanted advertising e-mail that deluges inboxes with offers of extended auto warranties, shady pharmacies that require no prescription and a great deal of other scurrilous material.

“We are seeing viruses infecting machines to set them up as spam zombies,” Belthoff said.