It might be a sticking state problem. If you're using the IntelliType
keyboard drivers, sometimes the Ctrl (as well as Alt, sometimes) key state
gets stuck (has nothing to do with hardware, but is an interaction between
the IntelliType drivers and IE7, so you're more likely to see the problem
when switching from IE7 to another application).
When this happens, pressing ordinary letters then has the same effect as the
Ctrl+ versions of those letters until the stuck state gets cleared (which
usually can be done by pressing/releasing Ctrl/Alt/Shift at the same time).