<p>What do you use to validate an email address on a ASP.NET form. I want to make sure that it contains no XSS exploits.</p>

<p>This is ASP.NET 1.1</p>

Preventing XSS is a different issue from validating input.
Regarding XSS: You should not try to check
input
for XSS or related exploits. You should prevent XSS exploits, SQL injection and so on by escaping correctly when inserting strings into a different language where some characters are "magic", eg, when inserting strings in HTML or SQL. For example a name like O'Reilly is perfectly valid input, but could cause a crash or worse if inserted unescaped into SQL. You cannot prevent that kind of problems by validating input.
Validation of user input makes sense to prevent missing or malformed data, eg. a user writing "asdf" in the zip-code field and so on. Wrt. e-mail adresses, the syntax is so complex though, that it doesnt provide much benefit to validate it using a regex. Just check that it contains a "@".