In the text, uninvited guests refers to

Old Tray, New Tricks: Windows 7's Taskbar and window
management tweaks are nice. But its changes to the
System Tray - aka the Notification Area - have a huge
positive effect.

Changes in Windows 7 transform the System Tray from
an intrusive eyesore (in Windows Vista) into a useful set of
shortcuts and other controls.
In the past, no feature of Windows packed more
frustration per square inch than the System Tray. It quickly grew
dense with applets that users did not want in the first place, and
many of the uninvited guests employed word balloons and
other intrusive methods to alert users to uninteresting facts at
inopportune moments. At their worst, System Tray applets
behaved like belligerent squatters, and Windows did little to put
users [PARTICLE] in charge.
In Windows 7, applets can't pester you unbidden
because software installers can't dump them into the System
Tray. Instead, applets land in a holding pen that appears only
when you click it, a much-improved version of the overflow area
used in previous incarnations of the Tray. Applets in the pen
can't float word balloons at you unless you permit them to do so.
In Windows 7, applets can't pester you unbidden
because software installers can't dump them into the System
Tray. Instead, applets land in a holding pen that appears only
when you click it, a much-improved version of the overflow area
used in previous incarnations of the Tray. Applets in the pen
can't float word balloons at you unless you permit them to do so.
It's a cinch to drag them into the System Tray or out of it again,
so you enjoy complete control over which applets reside there.
More good news: Windows 7 largely dispenses with the
onslaught of word-balloon warnings from the OS about
troubleshooting issues, potential security problems, and the like.
A new area called Action Center - a revamped version of Vista's
Security Center - queues up such alerts so you can deal with
them at your convenience. Action Center does issue
notifications of its own from the System Tray, but you can shut
these off if you don't want them pestering you.
All of this helps make Windows 7 the least distracting,
least intrusive Microsoft OS in a very long time. It's a giant step
forward from the days when Windows thought nothing of
interrupting your work to inform you that it had detected unused
icons on your desktop.

(Adapted from
http://www.pcworld.com/article/172602/windows_7_review.html)