One of the best (and probably most overlooked features of Vista) is the "Preview Pane" (in Explorer Menu: Organize > Layout > Preview Pane), which makes it really easy to see the content of files without having to open the actual Application (as a sidenode, I saw a video yesterday about the new Leopard OS which contains two "new" and "innovative" features called "Cover Flow" and "Quick Look", which is - yup, you guessed it - Preview of files directly in Explor... sorry Finder :-) .. so much for Apple comming up with new ideas!)
Anyway... if you're running Office 2007, you're able to preview Word, Excel etc. file formats (don't know if this also works if you're using Office 2003). Sadly a format which is not supported native by Vista is PDF.
Luckily Tim Heuer and Foxit (by far the fastest PDF reader for windows, check it out!) have joined forces and put together a PDF preview handler which can be downloaded right here: FoxitPdfPreviewHandlerSetup.msi (1,68 mb)
If you're into creating your own Preview Handler, take a look at the this article from MSDN Magazine.