Easy way is consult the MS site..
Rumors have been abound since Vista on how to upgrade. I have upgraded every version fo Win7 (Beta Test).
I could supply links. Yes, but it best info is FROM MS, not some 3rd rate magazine, that has no idea what Windows 7 really is... Most of them still use Win XP.
UPGRADE or Clean install from the UPGRADE Disk have been available since way back.
The provided Manual will give clear instructions, the same can be found on the Official MS site.
To prepare may customers for the official (release of Win7) while using XP.
1. take any Vista Disk, and upgrade the Same version from XP to Vista,
NO I HAVE NOT USED A KEY. YOU HAVE 30 DAYS TO ACTIVATE. AND IT IS FULL.
2. Take you download version of Win 7 RC-7100 (official release from MS), it is FREE on the MS Site. obtain a Official KEY. then insert the DVD while logged in to Vista, click upgrade, when ask insert the Official KEY.
then you have enough time to wait until your customer - etx whom ever, gets there Official RTM Disk, either (UPGRADE or RETAIL or OEM) and then insert the DVD while booted and UPGRADE and insert the KEY when ask.
(NOW THIS REQUIRES THAT CHECK THE OFFICIAL MICROSOFT SITE) to find information on exactly which versions can be legally upgraded.
Some version require only a CLEAN install while other version allow UPGRADE.
I recommend you visit and browse Microsoft.com - MDSN - Technet for appropiate Offical information.
REASON: every body has their oipinion. and unless you read it from the Official Site, you will not fully understand.
Official Beta Tester for Microsoft since Pre-Windows 95.