have you tried uninstalling it in safe mode? maybe this will allow it to uninstall.
If it still wont budge when you try to uninstall it from safe mode then you will need to delete the ubisoft game folder in C:\program files or the location you saved it to and then edit the registry to remove it from the add remove programs list. a bit messy but it should work in my opinion. make a system restore point if you are trying this just in case.
right click your 'start button/explore' then navigate to the ubisoft far cry folder and delete it.
next click 'start/run' and type 'regedit' then click ok. click on 'my computer' (top left) to highlight it and then click 'file/export' this will allow you to save a back up of your registry to a location of your choosing that can be restored by clicking on the .reg file (just in case things go bad and system restore lets you down)
once backed up click the '+' next to 'HKEY_LOCAL_MACHINE' to expand it and navigate to 'Software\Microsoft\Windows\CurrentVersion\Uninstall' click the '+' next to 'uninstall' and you will see a list below it of all the software listed in add remove programs. look here for the entry related to far cry or ubisoft and delete it. restart your computer.
ps. the above method only removes the entry from add remove programs so i would run a reg cleaner to clean up the rest of the reg entries left behind by the game if you wish (shouldnt do any harm to leave them though. regseeker is easy to use click here