Allo Tor,
if you haven't done so already you need to first do this:
1 Click:
Start-> Connect To -> Show all connections
2 Once your in there, click on the option Create a New Connection at the top left under "Network Tasks"
3 Now you should see "Welcome to the New Connection Wizard" - Click Next
4 Ensure "Connect to the Internet" is selected - Click Next
5 Select "Set up my connection manually" - Click Next
6 Select "Connect using a broadband connection that requries a user name and password" - Click Next
7 Enter anything you like for the "ISP Name" field, better to simply call it Arcor in this case -> Click Next
8 Create this connection for: "My use only", unless you have other users logging on to this machine who you want to let connect to the internet using your internet connection; if you do then select "Anyon's use". But for now let's just use "My use only" for security reasons. -> Click Next
9 Internet Account Information ; in this message box you give in your Arcor Credentials. Type in your credentials and click Next
10 If you require a shortcut on your desktop, which is pretty handy for beginners or if you have a short memory because you drink too much alcohol

then click the checkbox next to "Add a shortcut to this connection to my desktop" - Click Finish
11 Try connecting to the internet by double clicking on the Network Connection you just created.
12 If you cannot connect then post the errors you see in this thread. It is possible that we have to configure and then setup your router. (If it's WLAN then we have to configure it differently

).
Good luck.
By the way, Arcor is a very good choice once your up and running it always works.
T-Online is the biggest piece of shit available when it comes to ISP's, they have so many cutomers that they just don't care anymore.