I assume you mean ERRORLOG, ERRORLOG.1, etc. They're text files, any text editor or word processor can handle them

Double click the file. Windows will bring up a dialog that says

Select the "Select a program" option. On the dialog that then opens pick Notepad, click OK. Errorlog file opens in notepad.

If you don't want to permanently associate the .1, .2, .3 file 'types' with notepad, then uncheck the option at the bottom of the second dialog before hitting OK

The errorlog file (without extension), will bring up the Open With dialog directly (at least it does in Win7)