What kind of documents - ALL documents of every filetype or only a specific filetype (and, if so, what filetype)? When did this start? What programs are you using to try to open the documents (or rather, what programs do you expect the documents to open with)?
Go to Start / Control Panel / Administrative Tools / Event Viewer and click on the Application section to the left. Here's how to use Event Viewer: http://www.petri.co.il/vista-event-viewer.htm.
You should see a listing on the right containing events. Look for ones with Red symbols which seem to occur frequently - those will probably contain information about these errors. Double-click on the errors for more detailed information, copy them, and post them here in your next reply. Check a few to see if they say different things and, if so, post a few so we can see how the problem differs (if it does differ).
If you can't get into Event Viewer, try booting into Safe Mode (repeatedly clicking on the F8 button during bootup until the menu appears and then selecting it) and see if you can access it in that mode.
Knowing more about the error will help us determine the cause and see if we can come up with a specific resolution or options to try to find and correct the problem.
In the meantime, you can try one option that may work. Do you know when this problem began? Try a System Restore to a point in time BEFORE the problem began. Here's the procedure: http://www.vistax64.com/tutorials/76905-system-restore-how.html. Be sure to check the box to show more than 5 days of restore points. If the first attempt fails, then try an earlier point or two. NOTE: You will have to re-install any software and updates you installed between now and the restore point, but you can use Windows Update for the updates.
I hope this helps or at least helps narrow down the problem.