If you have a lot of ASP code which currently runs on a Windows 2000 server, you probably use CDONTS to send your mail...Now, if you want to migrate to Windows 2003 (or Windows XP for local dev) you'll have a problem - CDONTS no longer comes with those environments...
Luckily CDONTS still works, you just have to hunt down a file called CDONTS.dll in the system32 directory of your Win 2000 box, copy it to your 2003 box and register it...your old code will then work (though CDOSYS is better!)