<p>How can I validate a username and password against Active Directory? I simply want to check if a username and password are correct.</p>

Probably easiest way is to PInvoke LogonUser Win32 API.e.g.
http://www.pinvoke.net/default.aspx/advapi32/LogonUser.html
MSDN Reference here...
http://msdn.microsoft.com/en-us/library/aa378184.aspx
Definitely want to use logon type
```
```
LOGON32_LOGON_NETWORK (3)
```
```
This creates a lightweight token only - perfect for AuthN checks.  (other types can be used to build interactive sessions etc.)