Hardrive always changing space I have left

two things one: my hard drive keeps changing the space avaliable to use this mornin it said 102 gigs left a few hours later its 82.2 left, the other day it was 96 then after I rebooted for a system mechanic update it was 86. I tryed system mechanic and it said I have a hard drive error, but when I used other programs and scanned the surface and files they all said my hardrive is fine. the other thing I have vista 32 bit addition, do I have to reformat the drives t install vista 64 bit? I have 3 gigs of ddr2800 and im happy with the computer with my zotac pcie 2.0 9800gt overclocked and all, I was told by HP that theres still alot of issues with 64 bit with games so I was just asking about that as well, thank you for your input.