Hi. I was having a similar problem. After I installed today's windows update for Vista 64 (and rebooted) and my scan is going further than it had been going. This may have fixed my problem. I have a lot of drives and a lot of stuff though so i won't know if this fixed it for a while.
For any having the same problem:
Update for Windows Vista for x64-based Systems (KB979306)
Installation date: 2/23/2010 2:54 PM
Installation status: Successful
Update type: Important
If you have a tiny drive try it and let me know if i can look forward to this working please lol