This is how a Textbook Uninstall should go.
If you installed CTM alongside conflicting software, or have somehow otherwise borked your system, to the point that it will not boot, as in failed CTM uninstall, "operating system not found" or similar indication.
Odds are that your MBR has been corrupted.
See Part:B Part:A
To uninstall COMODO Time Machine
Click Start > Settings > Control Panel
In the Control Panel, double-click Add/Remove Programs
In the list of currently installed programs, click COMODO Time Machine
Click the 'Remove' button.
Click Start > All Programs > COMODO > Time Machine > Uninstall.
A dialog box will appear to confirm that you want to un-install COMODO Time Machine. Click 'Yes'.
A progress dialog box will appear, indicating the Uninstallation process.
You will be prompted to select the snapshot to which your system has to be rolled back before uninstalling the application.
Current System - If you want to keep the current state of the system intact and uninstall Comodo Time Machine, select the 'Current system' and click 'OK'.
Select one snapshot to restore to - If you want to roll back your system to a previous snapshot before uninstalling Comodo Time Machine, select the radio button 'Select one snapshot to restore to' and choose the snapshot you want to roll back to, from the drop-down menu and click 'OK'.
A progress dialog will appear.
On Completion, you will be prompted to restart the system for the uninstallation to take effect.
Click Yes to restart your system. If you plan to restart the system at a later time, click No. The uninstallation will be effective only on next restart.
On the restart, your system will be rolled back to the snapshot you selected and the Comodo Time machine will be removed.Part:B
Caution is advised from here on. Please try to verify with help if necessary that your MBR is indeed damaged/corrupted.
You can seriously hose a system with these tools.
For Vista and Win 7
How to use the Bootrec.exe tool in the Windows Recovery Environmenthttp://support.microsoft.com/kb/927392
For Windows XP
Description of the Windows XP Recovery Consolehttp://support.microsoft.com/kb/314058
an alternative utility for XP and older windows can be found here.http://www.ambience.sk/fdisk-master-boot-record-windows-linux-lilo-fixmbr.php
With this you won't need recovery console or windows disc, but you will need to get proficient enough to make a bootable floppy or usb drive work.
Ignore the names "linux lilo", It doesn't matter how it got hosed. The same fix applies.
This should be more than enough info. At least regarding getting back into your OS.
And as always just ask in the forum for help with individual steps, clarifications.