Hi,
I think I am getting a false positive. I am running Avast Home Edition on Windows XP.
Process Explorer is a legitimate Sysinternals utility:
http://technet.microsoft.com/en-us/sysinternals/bb896653.aspx
Today, when I started my computer Avast Home Edition detected one of its files (PROCEXP111.SYS) as a rootkit. I submitted the file to Alwil through the program window.
Let me know if you need anything else.
I would suggest a forum search for procexp111.sys as I’m sure I can recall this file name cropping up recently.
This is it, http://forum.avast.com/index.php?topic=34665.0, note there is a newer version of process explorer.
In that topic there is a shortcut link to a topic in the sysinternals forum about this also worth a read, http://tinyurl.com/5rska6.
Thanks for the reply. The idea of searching before posting escaped me completely…
Next time I’ll be more careful! 
You’re welcome.
There is a wealth of information on the forums, as you say you just have to remember ;D