I have seen quite a few cases that any call to method 'Post' of different objects such as 'IOWSPostData' (IOWSPostData.Post()) in Excel returns with failed dialog.One of the most frequent cases is when you try to import a list from an existing Excel file. This has nothing to do with WSS 3.0 but rather the result of a failed call in the Excel Add-in which results in Excel using the IOWSPostData.Post() method to publish the Excel range or query. These post calls have been around since SharePoint Team Services 1.0 days, so if you want to change it to a soap call instead follow these steps :
1) Go to C:\Program Files\Microsoft Office\Office12\1033
2) Open the Excel Add-In EXPTOOWS.XLA
3) Find Intialize() method
4) In Initialize mthod comment out that lVer = Application.SharePointVersion(URL)
5) Add the line lVer=2 instead
Your Intialize() method should now look like this:
Sub Initialize(List, Title, URL, QuickLaunch)
strQuickLaunch = QuickLaunch
aTarget(iPublishURL) = URL
aTarget(iPublishListName) = List
aTarget(iPublishListDesc) = Title
'lVer = Application.SharePointVersion(URL)
lVer = 2 'Or any number greater than 2
End Sub