Windows Vista offers a really nice preview pane in Windows Explorer. Using the preview pane, you can actually see a Microsoft Office document (spreadsheet, word doc, powerpoint, etc) in it's entirety without having to open the program itself. You can even copy content out of the previewed document so that if you need to get some numbers out of a spreadsheet, you can do so without ever having to open Excel.

If you aren't currently seeing previews in Windows Explorer, click Organize in the Explorer toolbar then select Layout, and then Preview. You'll then get a preview pane. Once the preview pane is shown, you can select the corner of it to resize the pane.