The Love Bug is more destructive than Melissa as it overwrites audio and picture files, replacing them with its own code.
The computer security firm Symantec says it is looking at 10 copycat viruses able to elude software designed to block messages with the original Love Bug virus.
One of the copycat viruses entices users to open an e-mail with the word "Joke" in the subject line that unleashes yet another virus masquerading as a "Very Funny" attachment.
The search for its creator, involving the FBI and Interpol, is focusing on the Philippines after a Manila internet services provider reported that the virus appeared to have spread from two of its e-mail addresses.
The bug has affected tens of millions of computers, according to some estimates, and caused damage worth hundreds of millions of dollars. Computers not running Microsoft software will not be infected.
California-based IT consultancy Computer Economics estimated worldwide damage to be $2.6bn by the end of Thursday. It said that figure could soar to $10bn by next week.
Dormant virus
Manila ISP Supernet said the original Love Bug e-mail was uploaded onto its servers as early as 28 April but remained dormant until Thursday.
"He's been very crafty. He's been very able to move around the network, the various ISPs, and he's been very hard to locate," said Jose Carlotta, chief operating officer of Supernet's parent company Access Net.
"He's been using a lot of hacked accounts ... he illegally gets the user name and passwords and he uses those accounts for his own purposes."
However, Mr Carlotta said it was possible the creator was in another city or country.
Lithuanian officials said on Friday they would investigate whether the virus might have originated there, after a Love Bug began circulating with the title, in Lithuanian, of "Let's meet this evening for coffee".
Like last year's Melissa virus, the Love Bug is spread by e-mails that multiply once opened by the recipient, sending new messages to everyone in the user's address book.
Rapid replication
In addition, the virus is spread through instant messaging systems which allow people to chat in real time on the internet.
The Love Bug is more destructive than Melissa as it overwrites audio and picture files, replacing them with its own code.
The virus is reactivated if one of these files is subsequently opened.
The proliferation of messages on Thursday created a huge volume of electronic mail which overwhelmed computer systems.
US government agencies including the State Department, the CIA and the Defense Department said their computer systems had been hit, but that top-security material had not been affected.
The State Department was forced to disconnect its computer systems from the internet, a spokesman said.
US companies were likewise forced to take their e-mail systems off-line to isolate the spread of the virus.
The Love Bug was blamed for shutting down the website of the state lottery in Florida.
One DJ in Texas received the e-mail virus 1,500 times.
Delete
Europe was also hard hit on Thursday, with the victims including the UK House of Commons, the Danish Parliament and the Swiss federal government, along with many banks and other companies.
About 10% of businesses around the UK were reported to have been affected.
Computer experts have advised people using Microsoft Outlook who receive the e-mail not to open it, but to shift-delete it immediately.
If the message is opened, users should on no account open the attachment that comes with it.
If the attachment is opened, experts advise users to log off, switch off the computer, and contact their help desk.