Unwanted automatic redirect to HTTPS

I have two domain names which are pointing to the same server.

1. example.com
2. example.com.sg

My problem is my client enters example.com as the URL on any browser, it loads properly. But when my client enters example.com.sg, it redirected to https://example.com.sg [<-- note the 'S'] with IIS7 page. (Browsers: Chrome, Mozilla Firefox and IE)

example.com.sg loads nicely on my computer(window 7 32 bit OS) and all of my colleagues computer(mac book). But only my client and some of her colleagues are facing this issue. They are using window 7 64 bit OS.

Then I tried to use URL example.com.sg/stage which is the new website I developed. But I get a 404 error.

I tried some solutions but not working.

1. Clear browsing data, empty the cache and delete cookies
2. Redirect code by using .htaccess file
3. Header cache control headers to 301

I don't understand why, whether is computer setting problem or other reasons? Hope some of you could provide me with some advice. Thank you.