Microsoft is to release six security bulletins as part of next Tuesday's monthly patch release. 

The update will include fixes for 'critical' vulnerabilities in Excel, .Net and Windows Server. Two of the remaining bulletins are rated 'important' and a sixth is rated 'moderate'.

Microsoft classifies each bulletin by its maximum risk level because a vulnerability can often have differing degrees of risk for each software version.

If exploited, each of the three 'critical' vulnerabilities could allow for remote code execution.

The first 'critical' vulnerability lies in Excel 2000 Service Pack 3. Other versions of Excel 2003 and 2007 list the vulnerability as 'important'.

Microsoft's .Net framework received a 'critical' fix for seven versions of the software, ranging from 1.0 to 2.0.

The third 'critical' flaw is in Windows 2000 Server SP4. Windows Server 2003 SP1, SP2, x64 and Itanium all classify the vulnerability as 'important'. Other versions of Windows are not affected.

The two 'important' fixes are in Publisher 2007 and Windows XP Professional SP2, while the lone 'moderate' risk vulnerability affects the 32-bit and 64-bit versions of Windows Vista.

Microsoft has touted the safety of its latest operating system, but this has not spared the software from its share of 'critical' security flaws.

Microsoft issued one 'critical' update for the Mail application last month that comes bundled with the operating system. 

The company is also planning non-security update for the Windows Malicious Software Removal Tool and the Windows Update, Windows Server Update Services, and Software Update Services.

The monthly patch is scheduled for 10 July through Windows Update. Users can also download the patch from Microsoft's website.