The license you bought for XP is a single
seat, not a site or house license. So you
bought a license for one PC, if you want to
run XP elsewhere you have to buy another.
Per seat licensing, or site licensing is
available to businesses, but not
individuals - probably not much of a market.
Anyway you agreed to that when you installed
or activated XP, the agreement says it in
the legal disclosure. Additionally you
don't actually own the software, you're just
paying for the ability to use it, which can
be revoked. This is a standard licensing
agreement, not something Microsoft cooked
up. Symantec, McAfee, Adobe, etc., all have
such standard EULA's (End User License
Agreements).
As for upgrading an existing system - you
can avoid issues by wiping your hard drive,
re-installing Windows 98, ME, etc, and then
installing the XP upgrade. The issues
arrise when there are problems with your
current installation of Windows 9x you may
or may not know about, that are passed on or
cause more problems during the upgrade.
The price of PC's keeps falling, hopefully
the prediction will come true that you'll
buy the OS and they'll give you a PC to go
with it. You can find a sub $300 PC now, I
wouldn't try playing the latest graphic
intensive games on it, but it would be fine
for surfing the Internet, controlling X10,
light gaming, doing your checkbook, etc. As
they get cheaper it'll be like buying
another appliance for your home.