Like I said ... don't run Windows ... but ... 2.6 to 2.7 won't do and the errors you show might indicate you've already upgraded the PHP to a higher version in anticipation of being able to hyperjump. When it comes to upgrading from an old to highest, all platforms are in the same boat. You can't pull the trigger on upgrading PHP until you are sure the version you have can run the upgrade to the version you are moving to. That make sense?
Here's why I think the above ...
add_to_log() has been deprecated
So you might have to down grade your PHP so that the uprade to 2.7 will work.
PHP 5.4.x or 5.5.x on Windows.
So let's say that works and you do get to 2.7.highest.
Rather than searching the web and a lot of scratching of heads.
Go to Site Admin -> Server -> Environment.
Update the component
Then use the drop down pick list for moodle version and see what Moodle says you'll need ... for php version, for php extensions, for DB config, etc.. You should be able to map out a plan then.
@Tunde ... seldom does one have *exactly* the same problem ... *same exact error* ... but yours might be *similar* to Katherine's. Anyhoo the same advice ... use the Environment check on existing version to see what you need moving forward from an old version to map out when to upgrade PHP, etc..
'spirit of sharing', Ken