Been having this problem too for a while and it always happened on Sunday somewhere between 12-17h.
If Vista was on during those hours, I was 100% sure I would have to repair GRUB.
It would just stay on "grub loading" for a couple of seconds and reboot.
All the other days seemed ok, even when leaving windows for a long time... Tried the same routine on Saturday/Sunday and only Sunday gave me problems somehow.
Tried removing some Dell software in a couple of steps (like that ISUSPM.exe), but still the problem persisted.
Dell Vostro 1720 - Vista/Ubuntu 9.10
Been having this problem too for a while and it always happened on Sunday somewhere between 12-17h.
If Vista was on during those hours, I was 100% sure I would have to repair GRUB.
It would just stay on "grub loading" for a couple of seconds and reboot.
All the other days seemed ok, even when leaving windows for a long time... Tried the same routine on Saturday/Sunday and only Sunday gave me problems somehow.
Tried removing some Dell software in a couple of steps (like that ISUSPM.exe), but still the problem persisted.
Last week I updated my BIOS from A02 to A08 (a bit like Johnny Dopefish: https:/ /bugs.launchpad .net/ubuntu/ +source/ grub2/+ bug/441941/ comments/ 117) and it seems like the problem is gone for me now.