Application Not found CTFMON.EXE

Hi,
I have been using XP for some time and it has been OK.
This week that has changed. It has slowed down and is acting strange. My first thought was a virus.
I have AVG free and feel protected but now I don't know.
I have scanned the computer (not the one I am using now) and AVG see's no threats.
One other thing when booting a DOS window appears briefly, it says CTFMON.EXE in the corner and it dissappears.
The computer seems to be working a little better but when using the browser, often I cannot get where I want, I just get the message application not found. ie when I tried to access BBS all I got was application not found
Are there any Ideas out there,
Thanks.
Lewis