It may have something to do with Shadow Copying. You could turn off system restore but I suggest you make an offline backup of your files first. Once you have offline copies, ‘play around’ with the shadow copies, i.e. right-click a file and ‘Revert…….’. It might help to make up a small, unimportant file, alter it, revert to it, etc.
When a file is altered, the original version is stored as part of Vista's 'Shadow Copy' (part of System Restore but not present in the Home versions). Shadow copying is stored in System Volume Information (hidden folder) but you will have great difficulty identifying a specific file. If you don't have Shadow Copying switched on, the 'Revert to previous versions' may well be present as a selection but it will not find anything.
You have to turn on System Restore, here's a little explanation of Shadow Copying