For those who are not in the education market, and who are not buying multiple software licenses buycheapsoftware.com is a reliable source. Recent pricing:
Windows XP Professional with Service Pack 2: $138.00
Norton Antivirus, retail version $39.00
Office Small Business $219.00

They offer “OEM” versions of software, meaning just the disks without retail packaging or documentation. This is the software that would be included by a dealer when shipping a new machine.