No. My hands never rest on the keyboard (good old high school typing class).
Also, it happens when I'm not even close to either of those keys, and
sometimes it happens when I'm not typing.
The only explanations I can come up with for this type of behavior are
1) unintended input causing a switch to other windows (already posted). To
expand further, may want to check if the mouse buttons are sticking or set
to sensitively. Also any conductive material that has worked its way under
the keys (snip of cellophane or a bit of hair) could cause incorrect input
signals. Also wouldn't hurt to check cables of hardware to be sure that
they are not damaged.
2) If using wireless, another nearby device is causing problem input.
3) Applications stealing focus. Example, clicking away from a slow loading
web page and doing something else. When the web page finally loads, it pops
to the top. TweakUI for XP has a setting for adjusting this. Personally, I
use the setting that blinks the taskbar button of a "ready" program instead
of allowing it to steal focus. TweakUI is one of the Power toys for XP. A
link to the download can be found here:
http://www.microsoft.com/windowsxp/downloads/powertoys/default.mspx
Also related to the focus stealing... Check the system for spyware. It's
presence and operation may be interfering with your normal work.