http://windowsupdate.microsoft.com
Windows 2000 Compatibility Updates
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Windows 2000 was specifically architected to enable customers to easily add additional application support after the product had shipped for general release. The August 2000 package is the third release of such additional application support and includes software compatibility updates for Windows 2000. The August 2000 package includes the compatibility updates of the May 2000 and February 2000 packages
The readme on the web site lists specific applications that are affected by these updates. Interesting. I don't see Half Life specifically listed, though that may not mean much. The changes provided by these updates may offer functionality that helps Half Life in any case. I haven't gone to Sierra's web site for confirmation so couldn't say for sure whether these updates have anythig do with anything.
If these updates are not the case, then another reason for Evil69er's messages would be go along with what Shukaido stated above. But to further explain... The error reporting mechanism used by Half Life could be misidentifying the OS and the OS SP level because of a registry key or file check or another reason.
These two explanations make far more sense than that Win2k was running an NT SP level.