Hi,
If you have 1.5.3, you are probably not going to see enough difference between 1.5.3 and 1.5.6 to bother upgrading. There are no functional differences and only some minor bug fixes that apply to specific computers. So I wouldn't bother, especially if you don't have dialup.
The message that says it is not a valid win32 app generally means that the download is somehow being corrupted. You can order a CD of 1.5.6 from our a link on our downloads page, that might be your best bet if you have download, or you can download it at an internet cafe, your office or other location with a broadband connection and then copy the installer to your PC using a CD or a USB flash drive.
Thanks,
Don