Microsoft has issued patches for more than 20 flaws in its software, three of which were ranked ‘critical’.
In its monthly security update, the firm urged users to fix the vulnerabilities, which allow hackers to manipulate personal data via denial of service and remote access attacks.
The largest patch attempts to resolve 14 security issues, which affect versions of Microsoft Windows NT, 2000, XP and Server 2003.
Security firm Internet Security Services discovered one vulnerability in Microsoft's secure sockets layer library (SSL), which could leave thousands of online trading websites open to attack.
One of the patches is designed for vulnerabilities in the email program Outlook Express.
Several of the security issues make Microsoft operating systems vulnerable to viruses and worms, similar to Blaster, which infected thousands of computers last year.
Microsoft has published seven critical patches since its update scheme began last year, and 14 in total.
The company is expected to release Service Pack 2 for Windows XP, a generic security update, later this year.