There are quite a few online labs for Office, but I thought I'd point out this one in particular: Programmatic Manipulation of the Microsoft Office Open XML Formats Virtual Lab
I have to admit that I haven't tried this one out myself yet, so I'll be really interested to get feedback from folks who give it a try. I remember being asked to give feedback on it awhile ago, and they gave me a basic description of what it would cover. That was actually a good while ago though, but I do remember that it looked pretty good. If there is content missing, let me know.
If you like it, you should check out this link for a list of other Office labs: http://msdn.microsoft.com/virtuallabs/office/default.aspx Notice that there are a few labs that show you more about the XML support in Office 2003 as well.