I am not trying to sound condescending here but a Form Validation script IS what you want. (I am going to assume that you have basic understanding of ASP)
The username and 2 password fields are trivial because the username and password have to be something other then "" and the 2 passwords must match. (You need to determine whether or not you are making passwords case sensitive or not) In the case that passwords are not case sensitive you would do
Dim pw1, pw2
pw1 = Request.Form("pw1").toLower()
pw2 = Request.Form("pw2").toLower()
if pw1 <> pw2 then 'do something
The email address validation is a bit tricky (in .NET we have the benifit of a regex engine so it is a trivial matter to validate an email address) so i suggest you check out this link as far as email address validation goes:
Lastly, to make sure that the username the user has picked isnt in use you can do a couple of things:
SELECT [something] FROM [table] WHERE username=' & Request.Form("username") &"'"
The above example you check to see if something is return to your record set...i dont like that way of doing things, I would probably do:
SELECT count([field]) FROM [table] where username=' & Request.Form("username") &"'"
If this returns 0 the username doesn't exisit else it does.
I will only tell you how to do it, not do it for you.
Unless, of course, you want to hire me to do work for you.
^^Thats my signature