TQIT is checking your registry to see where vanilla TQ is installed, since you didn't "Install" it on that PC, your registry was not populated with the correct values.
This is my registry export:
Code:
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\Iron Lore]
[HKEY_LOCAL_MACHINE\SOFTWARE\Iron Lore\Titan Quest]
"LastMutexOwner"=dword:00000001
"CD Key"="***********"
"Install Location"="c:\\program files\\steam\\steamapps\\common\\titan quest"
"version"="1.300000"
[HKEY_LOCAL_MACHINE\SOFTWARE\Iron Lore\Titan Quest Immortal Throne]
"RanOnce"=dword:00000001
"CD Key"="************"
"Install Location"="c:\\program files\\steam\\steamapps\\common\\titan quest immortal throne"
You may have to replace the ********* with your actual CD key, I edited mine out, for obvious reasons :P
You will need to have those values in your registry and the filepaths correct for it to work on a HDD, copy that code up there, and paste it into notepad (or any text editor), edit the filepaths so they are pointing to the correct place, then save it as "tqit.reg", without the quotes of course. Then run the file, and it should put those keys in your registry.