Problems upgrading from 4.0.1 to 4.2

[ultranet]

I've spent hours now trying to upgrade from 4.0.1 to 4.2.
I've made a copy (well, about six so far) of my 4.0.1 installation. Then I've uploaded the upgrade patch, uploaded a new copy of the database, then applied the patch.

1. When I tried going from 4.0.1 to the latest 4.2.1, I got the following error:
******
Scanning Files...Including: cache/upload//upgrades/temp/b2DswG/scripts/post_install.php

Upgrading the database to version 4.2.1...

Running SQL file 401_to_421_mysql.sql...
An error has occured while performing db query. See log file for details.
Query Failed:-- files ALTER TABLE `files` ADD COLUMN `date_modified` datetime NULL FIRST;::MySQL error 1060: Duplicate column name 'date_modified'
******

2. I also tried just upgrading to 4.2.0d, and got another error which I have to confess I forgot to copy.

My impression is that the problem occurs with the db and versioning. How do I upgrade the db itself to 4.2.x?
Incidentally, I managed to install and upgrade on my Windows/XAMPP test platform first time, no problem.