I had a similar problem a few years back on xp, when I installed a certain very aggressive dvd-player software. The weird thing was, everything showed fine within the bios and windows, but when burning it was taking 30 minutes to burn a dvd, while this was an average 5-9 mins before. I found out about some errors in regedit, but no way could I set those straigth again. Only solution for me at the time was a clean re-install of winxp.
If you have the possibility, you could install winxp or vista on another hd and check if the drive works ok. You can unplug the hd you're using now or do a dual boot (though dual boot with vista uses very new ways to create a dual boot and is quite complicated to get rid off, once you decide to go back to single boot, as I underwent just a week or so ago.) If you still have trouble, than most probably it's the drive, but if it works well, you could chose to reinstall vista or to repair it or something. I don't know how recovery or restore works with vista but you can find that out easily by googling for it I guess. Good luck!