You should probably check your system for viruses and other nasties. Run a full virus scan with Norton, McAfee, or whatever program you use, making sure that you've updated it with the absolutely most current virus definitions.

After that, have a look through the threads in our Security forum to find out how to download and use utilities such as Spybot Search & Destroy, Ad Aware, and HijackThis. Run those utilities, and if they do find "spyware" and you have any further questions about that, start a new thread in the Security forum, giving us as much info as possible in your post. (Please do not post any virus/spyware/etc-related questions in any forum other than the Security forum.)

Do you get any error messages or anything when this happens?

I'd almost be willing to bet that it is a virus. I know that there was a virus or spyware something or other that actually named its payload notepad.exe. If you ever had this virus, that could be contributing.

Do you have a virus scanner?

Yup, Alex is right- the Qaz virus/trojan does that; here's more info from Symantec (Norton):

http://securityresponse.symantec.com...llw.qaz.a.html