Microsoft has fixed a bug in its Internet Explorer browser that led some experts to recommend people use a different way to surf the web.
The download.ject bug was spotted in late June and allowed infected machines to be controlled remotely.
Code to exploit this bug was somehow inserted on to some popular websites, infecting everyone that visited.
Until now the only protection against the bug were tools to detect the malicious code and remove it.
Microsoft said the patch had taken a month to finish because it wanted to be sure that it did not cause more problems than it solved for users.
In the same security update Microsoft is also patching other holes in its browser that could be exploited in the same way as download.ject.
Microsoft urged users to download the patch and apply it as soon as possible.
Those struck by download.ject risked having confidential information, such as passwords and credit card numbers, stolen from their computer.
Security experts recommended that people use another browser because of the novel way that the malicious bug spread and because Microsoft had, until now, no way to protect against it. However, soon after the download.ject bug was discovered the main danger was defused when Russian server dispensing the malicious code was shut down.