- The Upgrade versions require you to keep track of your original Windows XP/2000 disks. Most people have these in the form of "recovery CDs" from the PC vendor, which can include multiple disks full of junk applications.

- Using a copy of XP or 2000 as proof for the Upgrade version of Vista invalidates the XP key, according to Vista's End User License Agreement (EULA). The EULA states, in part: "Upon upgrade, this agreement takes the place of the agreement for the software you upgraded from. After you upgrade, you may no longer use the software you upgraded from." Some bloggers and newsgroup posters have speculated that you may not be able to use that "invalidated" XP license even for a dual-boot installation with Vista. Computerworld has contacted Microsoft for clarification on this and, at press time, has not received a response. In other words, this is yet another point of confusion about Vista. [Editor's note: Computerworld will provide an update when this information becomes available.]

- Many users have lost, or were never provided with, installation disks with their PC. Because they have XP or 2000 installed, they may decide to save money and buy an Upgrade version. If their disk later dies, or they need for whatever reason to reformat, they will then have to buy a second copy of Vista, this time, the full version. Ouch!

In a few years, future PCs may have hardware components not supported by XP or 2000. If a user buys the Upgrade version now, then later buys a PC and chooses to transfer the Vista license to it, the XP/2000 installation required by Upgrade versions of Vista may prove troublesome.

There is a widely published workaround that enables users to install Upgrade versions of Vista without XP. It involves, essentially, installing Vista twice. You can find the work-around in Computerworld's comprehensive Vista Upgrade Guide. Whether this work-around is considered by Microsoft as legitimate or a form of piracy -- like so much about Vista -- is still unknown.