In P4Merge if you notice that you cannot paste the text after you have selected it and done a copy via control+c or edit/copy, you might be affected by an impeding registry entry and a dll.
NOTE: This solution has worked well to fix this P4Merge behavior, but we do not guarantee it's effect on other software you may have installed.
The registry entry is:
and the value is:
This dll is a user mode executive module helper owned by Computer Associates
and located in c:\windows\system32\umxsbxexw.dll
If you remove the registry entry, the copy/paste feature will work with P4Merge.