Security changes in Windows XP Service Pack 2 will plug a gap that has previously allowed viruses to spread.

The built-in firewall in XP has now been set to activate by default before the PC joins the network.

Previously, viruses were allowed to spread within a company because the firewall did not start operating until after the PC had signed onto the network.

Changing settings has also been made easier for users, and administrators can use scripts to set large numbers of clients to block fast-spreading worms.

Multiple policies will also be set to allow different firewall settings at home or in the office.

Service Pack 2 is also designed to assist a new patching system, which could reduce the size of patches by 80 per cent.

New patch downloads will just include changes to code and not existing software, which will be useful for home users on dial-up connections.

Microsoft has also indicated its aim of making all future patches capable of being uninstalled in case they cause conflicts within a network.

Outlook Express and Windows Messenger have been strengthened to deal with attachable viruses, and Internet Explorer will block pop-ups by default.

Eighty per cent of the changes contained in the upgrade pack are focused on security, with most of the improvements involving new code rather than alterations.

"This is quite a step up from any service pack we've done before," said Paul Randle, Windows client product manager. "The security environment is changing and our code is changing to meet it."

The company stated that it had "extremely aggressive" plans for the business and consumer roll out of Service Pack 2, but declined to give figures.

While a release date has still not been fixed, the end of July or beginning of August looks likely.