Well, it's more complicated than that. You need a VPN server and you need to make a VPN tunnel. You also might want to use windows active directory. This all comes in windows server 2003, just need to configure it. unless you want to pay a ton of money, don't use citrix.

I'm just thinking, if there's just a certain program they need to access, why not use remote control software like logmein? You can probably save some money that way. What it does is it lets you access a computer from a web browser, so it's like you were sitting in front of it.

So I will need to get a window server software, and a logmein software.
Then i apply for an IP domain to link my server to the internet and all my users all over the world can access my software inside my server as long as i give them rights?

In terms of domains, you don't even need to spend a penny, as long as you don't care about a pretty URL. At DynDNS.com, and a few other sites as well, you can get *.oneoftheirdomains.com, as long as it's not taken, so for example you can have "iamsocool.is-a-geek.net", and they don't charge anything for the domain.

Additionally, you can use it with dynamic IP addresses, so you can get software that automatically updates the domain to point to your new IP address when it changes.

No, you will not need windows software. You will have to have each person that needs to access the computer remotely to go to www.logmein.com, create an account and install the software on their machine at work. Then to access the computer, they will need to go to the logmein page and log in from there. It's very simple, and it's also free.