Welcome to TSG!
On the same screen with the Safe Mode options, you may have an option to
Disable automatic restart on system failure
If that is there, it should prevent the restart, and instead give an error (Blue Screen) that might help pinpoint the problem.
If that option is not there, you can boot with the
Ultimate Boot CD for Windows and edit the registry to disable the automatic restart.
You'll want to set (or add) the
AutoReboot value to 0 under this key:
HKEY_LOCAL_MACHINE\SYSTEM\ControlSet
XXX\Control\CrashControl
The
XXX will be a three digit number
00X. Look under
HKEY_LOCAL_MACHINE\SYSTEM\Select for the
Default value to get the last digit of the controlset hive the system will use to boot normally, or just change all ControlSet
XXX trees that exist.
There may be *nix tools that you can use as well once you get write access to the Windows partition from your Ubuntu partition.
If you have your data backed up, a repair install may be faster than trying the above and then troubleshooting the problem though.
HTH
Jerry