Well, depends which route you wanna take :
The Windows route :
Windows 7/Home/Pro/Ultimate
Microsoft Office 2007/or beta of 2010, it's free to use for 6 months (please correct me if I'm wrong)
Microsoft Security Essentials
an Adobe Acrobat reader/freeware PDF reader
TuneUp Utilities 2010 (personal choice)
+ all the patches since release date.
The Linux route :
Wait till 29 April 2010 and get yourself Ubuntu 10.04.
It includes free Office software + everything else.
There is a link somewhere of all the "missing" software to install, once you have installed Ubuntu on your pc.
Enjoy building your new pc!!