Google improved the attachment viewer in Gmail. You can now directly preview a Microsoft Word document:
If you receive Microsoft® Word files as attachments in Gmail, you can now view them with a single click — no need to download, save, and open files with a desktop application when you just want read them. The Google Docs viewer that allows you to view .pdf, .ppt, and .tiff files in your browser now supports .doc and .docx formats too. Just click the "View" link at the bottom of a Gmail message and the viewer will take it from there. If you decide you want to edit the file, clicking "Edit online" will open it in Google Docs, or you can download it to your desktop from there.