Note'pad' is a default text editor that comes with Windows, that is best to use when editing the avast4.ini file. It is important to use a text editor rather than a word processor which can enter style/layout/formatting characters when editing, that is why I mentioned using notepad.
If the syswow64 doesn't exist (rather than unavailable, check using windows explorer) it is unlikely you have the 64bit version of Vista. If you actually don't know if you have it, then that is likely that you don't have it but the standard 32bit windows vista edition.
Check out the My Computer, System Properties, as that would say what Operating system version you have. See image example of mine, note I have windows XP Pro the standard 32bit version, so there is no mention of the 32bit, if that were the 64bit version it should show that also.