[crunkboy72]

Sorry if this is incredibly basic but how would I go about attaching one of those fake notepad error messages to a program like skype?

[Lynx]

A notepad error? Do you mean a message box?

I am no expert at this field but I guess you could change one of the dlls and make it run whatever you want. You'd simply need to swap the old one with the new (malicious) one.

[tux1]

Probably the easiest way is to make a fake error message using c# or vb and then bind it to your file of choice.