How did you upgrade Windows? Did you do an inplace upgrade or a new install?
What did you upgrade from and to? IE Windows Server 2003 to Windows Server 2008 R2.
Have you checked the mail settings under Admin/Settings.aspx?
To run it in debugger you'll need to download and install a version of Visual Studio (Web Express should do).
Then you use Visual Studio to open the Web Site, this opens SlickTicket in an editing view, and when you Debug it and replicate the situation that gives you an error. Hopefully it will throw an error and give you a better reference point for help.
PLEASE NOTE: If you are considering doing this I would recommend making a local copy of your website (copy the SlickTicket folder and paste it on a local drive to your workstation). I would also advise that you make a local copy of the database.
To do this you'll need SQL 2005 or 2008 installed on your local workstation, the Express Edition should be fine. Once that's installed you should create a new database named "SlickTicket", back up the server version, and restore it over your
local version. This way you don't accidentially make changes to your live web site.
With all that said, I'd check the settings page.