Well the first thing I would do is change your Windows Live ID password by clicking on the “Reset Password” link. Or you can obviously do this directly from inside your Windows Live account. Have you already done this?
Then let’s take it from there. I’m curious what indicates that your computer that was stolen is currently online? Why can you not open the files? Were they saved in a newer version of Microsoft Office than you are now using? If so, you will probably need the Microsoft Office Compatibility Pack for Word, Excel, and PowerPoint 2007 File Formats. If this helps then great, if not, please post another more detailed report and I am sure one of us can help you through this.
Brian K. Woytovich (Treebreeze)