in your email program, not Windows itself.
If I understand you correctly, you have a web site and you want to use that web site's email account as your default email account, right?
No matter what email program you use, you can set a default account, which will open when you click a web link. (I don't know what program you're using, so I can't give you specific advice on how to do it. Check your email program's help file on setting a default account. If you can't find anything in the help file, look for something like "tools" or "account settings" or "preferences" - you'll be able to set a default in one of those areas.)
For example, I use Netscape as my email program. I have 5 accounts configured, but my "Number 1" account is the default. I can change it at any time simply by changing the account settings. When I click on a web site link, an email window pops up using my "Number 1" account. (Sorry if I sound too "basic" - I don't know how advanced you are with computers, so it's better to explain things in too much detail than not enough. :-) )
If you use web based email accounts, how you set your default account depends on what web-based email you're using. If you have Yahoo mail, follow the instructions given
here. (I have a Yahoo account, so I checked their help file for you.) If you want your default account to be a another web-based email account, like Hotmail or Netscape WebMail, you'll have to visit their sites and check the help file - if you can't find it, search the help file for "default account" and it should direct you to the instructions.
cosmicaug was right ... we need more information to give you specific instructions, so if my post doesn't help you, let us know what program you use for email and we'll get you up and running. B-)