What can cause this?

You should not have a C:\explorer.exe file - it normally resides in C:\Windows\.
To enable us to help you get hijackthis:
-install it to a new folder alongside your program files.
-in that folder start HijackThis by dclicking the .exe; now close ALL other applications and any open windows including the explorer window containing HijackThis.
-Click the Scan and Save a Logfile button. Post the log here.
Better still, post it over in the viruses n spyware forum, cos that's where it should go.

It is C:\Windows\explorer.exe. My shorthand was too short I guess.


Yep. Okay, it will be a process or thread that explorer is handling, and a third party one at that, most likely, that has allowed the buffer overrun. [pretty much, a variable that was accepting input tried to pass on to its buffer more info than it could handle -the overflow "characters" disrupt following script, but there are other scenarios.].
Hackers love this method as a point of entry to your sys, they search for vulnerable processes - there is a v good chance that you have malware running already.
Seriously now, try what i posted earlier. It may be a poorly written app you have just loaded... but it may be a trojan etc trying to modify explorer... via a poorly written app.