The location of the WoW combat log depend on a lot of things, but one sure way to find it is with Process Explorer from Microsoft.
1. Download it from http://technet.microsoft.com/en-us/sysi ... 96653.aspx
2. Run the program. File -> Show Details for All Processes
3. Start WoW, type /combatlog to open the file
4. Go back to Process Explorer, press Ctrl-F, Search for C
ombat. Make sure you capitalize the C, it's case sensitive!
5. You should get something like in the screenshot.