I'd rather not have to set up a dual boot if I don't have to, but I'll keep it in mind.
Didn't get anywhere googling the error. Decided to try the ftp approach using the Filezilla that came with Xampp and created an anonymous user. Got a new screen after entering the credentials, which I've screencapped and attached. I'm assuming that's supposed to be a TOS screen which isn't showing quite right. After clicking the 'Agree' button, I got dumped back to the network install screen with no onscreen errors. There were some errors in the system error log though:
2013-06-11 8:01:23 - warning: AbanteCart core v.1.1.4 copy(C:/xampp/htdocs/abantecart_mantismoon/public_html/admin/controller/responses/listing_grid/customer_transaction.php): failed to open stream: Permission denied in C:\xampp\htdocs\abantecart_mantismoon\public_html\core\lib\backup.php on line 189
2013-06-11 8:01:23 - Error: Can't move file C:/xampp/htdocs/abantecart_mantismoon/public_html/admin/controller/responses/listing_grid/customer_transaction.php into C:/xampp/htdocs/abantecart_mantismoon/public_html/admin/system/backup/abantecart_1.1.4_2013-06-11-08-01-21/code/admin/controller/responses/listing_grid/during backup.
There was also a critical message about a backup error which more or less echoes the log entries above.
I've also attached the Filezilla log. The anonymous user's home directory is set to c:\xampp\htdocs\ with all the permission boxes checked (read, write, +subdirs, etc), but FZ appears to be looking for directories in the wrong place (unless I'm just reading the log wrong).
I also took a look in the install/Upgrade History, and it appears that the upgrade to 1.1.5 was partially successful as there are a number of upgrade type entries concerning version 1.1.5 core files and at least two backups completed by the package installer.
I'm thinking there's nothing seriously wrong, it's probably just a wrong setting somewhere?