Installing Games on Windows Vista+
It is strongly advised with any version of the Microsoft Windows operating system from Vista onwards, to install all games (including those run via Steam) to a games folder (such as 'C:\Games') outside of the 'Program Files' [and 'Program Files (x86)' on 64-bit hardware] folder trees.
Why, you ask?
Starting with Vista, after decades of telling developers to put everything into 'Program Files' folders, INIs, and the registry, MS changed the rules in response to malware making a mockery of their 'system security'. So in response they locked down those same areas they have been telling developers to utilize. Consequently to make changes to anything in the 'Program Files' folders, or INI files, or the registry, you had to have 'administration credentials', which they have for the same decades been telling everyone to not use for routine, daily use. Now, whenever a program tried to update the it's INI file, the UAC popped up and told you 'Uh-uh, tell me the password for an administrator (admin) account first'. But only programs written specifically for the Vista and later environments were prepared to deal with such an interruption to their control of all 'their' assets. Older programs just have fits. [Edit: Fortunately, most programs don't make changes to these areas after installation. But those that track 'status' in INI or other files in those locations don't fare well.] Games have additional problems because they often have to violate what MS considers 'good behavior' to achieve acceptable performance, which makes them look like malware in those 'secured' areas.
Some users think disabling UAC will solve the problem. That just disables the prompting to provide admin credentials. You still need to run as an 'admin account', which is suicidal if you have a connection to the internet. (See the definition of 'malware' and why MS resorted to locking things down in the first place. Think they won't bother with your home box? Ever hear of 'bot-nets'? Dozens of them, with millions of home computer slaves each under their control. Automatically probing and infecting any computer they can see. Routinely run as admin and 'resistance is futile; you WILL be assimilated', no matter what other precautions you take. They only have to 'get lucky' once and you have already given them the keys to the kingdom.)
Even when using 'admin credentials' you can still run into issues. The reason is that anything that is installed in the 'Program Files' folders are treated (whether flagged or not) as 'Read Only' by the system. Attempts to alter files located there are likely to be considered malware attempts. If you try to turn off the protection setting, it will reassert them the first chance the OS gets. If you do succeed in making an alteration to a file, the changed file is actually located in a 'shadow file' under your account name's AppData folder. A 'shadow file' is basically where the system redirects the request for the original file, as long as the approved API called are used. Games and applications do not always use the 'approved' API calls.
So, the preferred, least trouble solution is to install older programs, and especially games, somewhere other than in either of the 'Program Files' folder trees. On a completely separate drive (either physical or logical) is just fine, as long as that is where the app is installed. You can create a 'Program Files' folder on another drive and install all your older apps and games there without any of the problems with those folders on the C: drive. Don't try to move the programs there after installation without changing the registry. Much better to uninstall and reinstall in that case.
Solution for Steam Games
See Moving a Steam Installation and Games for the official Steam Support instructions.