I use Content-Disposition to open up a save as window, can I use an absolute path?

Now, I have header("Content-Disposition: filename=file1.txt");

I tried to do header("Content-Disposition: filename=C:\Program Files\file1.txt");

But, it's not working. The save as window pops up, but it doesn't give the file the right filename (which is file1), and it doesn't know the type (which is .txt)

So, we can't use an absolute path with Content-Disposition?