Uninstall it again and reinstall it. If it's the Gothic Universe collection, I advise you to go directly to the jowood fora. 'They' made it, so they know how they can help you the best.
If it's however an illegal distribution, I can only suggest that you buy a legal one.
How to manually uninstall Gothic...per Nico Bendlin (dev)
stop here if you are not familar with Windows and regedit
- Run RegEdit.exe and goto HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall
- search for 'gothic' - you should find an subkey where 'DisplayName' = 'Gothic'
- remember the GUID (the name of the subkey you found - {BBF10B37-4ED3-11D5-A818-00500435FC18} )
- delete the subkey you found
- delete the folder [programs]\InstallShield Installation Information\[GUID]
If you installed the patch - do the same for 'DisplayName' = 'Gothic_Patch'
Finally delete the gothic folder and the entries in the startmenu.