It's a Roy Rogers song from his film My Pal Trigger. The first part goes;
All the cowhands wanna marry Harriet,
Harriet's handy with a lariat,
But she don't want to marry yet,
She's havin' too much fun!
I don't know if Bob Nolan wrote this song or not. I think it's called "All the Cowhands Wanna Marry Harriet", but I've also seen it called "She's Havin' too Much Fun".
It's all over the 'lyrics' sites on the web, but those sites are just infested with viruses so I don't go on them.
There's also the song "Old Faithful" in the movie, but I should probably make a new thread for that if I can't find the lyrics.