Just in case you still have windows 2003, Now it is the time to upgrade.
Windows 2008, 2008 R2 and now 2012 are out, so jumping from 2003 to 2012 could men lot’s of extra work if you use Active directory and Exchange 2003.
There are some very good “how to upgrade” blogs/documents out there, but every case is a case, you my get stuck without any useful answers from those main stream “how tos” when you get some strange errors. Best Approach is to upgrade by steps, 2003->2008R2->2012.
Virtualize your Windows 2003 box and use a test environment to test the upgrade.