One of our users reported a problem where they could open documents in Read-Only mode OK but if they tried to open them for edit they got an error message box, " 'Edit Document' requires a Windows SharePoint Services-compatible application and Microsoft Internet Explorer 6.0 or greater.". They had IE 6 SP2 and Office 2003 so should be fine, but they were not.
After some research found Microsoft Knowledgebase article 833714 (and many forums and blog posts that linked to it) which contained a number methods to resolve this error message. We found that whilst their system claimed to have SharePoint support installed in the Office install it did not show as an add-on in IE. We removed and re-installed SharePoint support and they could open a document in edit mode.
We did also find a blog post that indicated that KB833714 doesn't always resolve this issue. The recommended resolution in that situation is to completely remove and re-install Office.
A number of the blog and forum posts identified the cause of the problem as the user has components from more then one version of Office installed (e.g. Office 2003 and OneNote 2007) and the older one installs a patch or update. This can result in older versions of DLLs being registered over newer versions. This may be the case here as this user had both Office Professional 2003 and Office Standard 2003, not exactly the same but close.