If you are looking for building a hosting environment, I'd recommend you to use any of the server OS (Win 2003 or Win 2008) since most of the services will be installable only on servers. Also, Win XP is more of a desktop OS. You can consider something as below:
Windows 2008 server
SmarterMail (Free version available with 1 domain/10 users limitation)
Microsoft Web Installer (installs everything including ASP.NET, PHP, MySQL, SQL Server)
You may also consider some of the VPS services available which will provide you with similar kind of environment.
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You'll probably find a few quality Windows companies who can help you get started with free control panel (such as dotnetpanel) and smartermail etc. Do a search for dotnetpanel hosting in google and you'll find a few candidates.
You can get help from any Data Center who sells Windows Servers. You can easily google for it. Regarding the Mail Server options I would recommend MailEnable Standard version which is FREE and has no restriction of number of maildomains OR mailboxes.
Also, for control panel I would recommend Hosting Controller as they have got all things which an ISP looks to offer to their clients.
There are free control panels but those are mostly Linux based but I think you want to start your Windows Hosting business. Is it correct?