Bad, but how much it affects you will be random.
It most likely means that it failed to cleanly finish off writing a bunch of files to the disk due to a power cut, software problem or a problem with one portion of the disk.
It all depends on what files were lost. If they're a bunch of drivers for hardware that you'll never connect to your machine, then you'll never even notice.
If they were some of your data, you may not notice until you go to open that spreadsheet in 6 months' time and find that it's gone/corrupted.
If it's a vital part of a program or Vista itself you may find out pretty soon.
99% of the time it'll be some useless cache file from the browser (since there are so many of them) and you won't notice any difference.
If it's taking a long time it could be a sign that there's a disk problem, in which case it should mark the appropriate area of the disk as bad and continue on merrily, or the disk could fail within days taking everything with it.
Got everything important backed up?
