Save installation date in registry

Does it have to be the date it was installed, or would it be good enough if it was the first time the progarm was run?

There's a code project article on reading/writing the registry from C#:

http://www.codeproject.com/Articles/3389/Read-write-and-delete-from-registry-with-C

If you just check for the existence of your registry value when the program starts up, and set it to the current time/date if it isn't there, then you can do it all in one place at the start of your program.

If you have to do it at install time, then you are going to have to have the installer write the value into the registry when it runs. How you do that is going to depend on what you are using for installation.