it's a bug.
we already fixed it in 1.1.6.
to quick fix replace file admin/controller/responses/common/zone.php by attached
and let me know

Please, use search by forum
http://forum.abantecart.com/index.php/topic,784.msg3151.html#msg3151

You have .htaccess from some other application that is conflicting with AbanteCart. Please make sure you use correct .htaccess

yes. your c:\xampp\htdocs\abantecart_mantismoon\public_html is document root directory of web-server.
When you enable ftp for upgrading abantecart will try to find it. It's also looking for directories with those names like htdocs, www, public_html

abantecart upgrade package DO NOT change any custom data. Only core's and default template files
We are always recommend to users do full backup of your site (files + mysql dump) before upgrade.
If you want to upgrade to latest version I would recommend to wait a bit  and do upgrade to version 1.1.6 too that will be released soon.

it's a bug. Commit with all changed files  here
https://github.com/abantecart/abantecart-src/commit/0ecf6ef979ea42327f7bf1b7475b977cf40962a2
you can just replace old files by changed.
Note: bug fix is only for v1.1.5 and goes to version 1.1.6 that will be released soon

try to search word "maintenance" in admin

regarding dual boot. You do not have to. You can to create bootable usb-flash drive and tune all what you need there, i mean l.a.m.p..
But agreed to create a testcase for the OS, which differs from the OS of hosting provider it at least strange.
Windows is bad os for realtime tasks, especially vista.

could you try to set your document_root directory(root of webserver) as home directory for ftp user ?
We tune ftp mode for common cases, by another words, for hosting providers. As i see in filezilla log abantecart try to find your document root directory bu ftp-search and not found directory that  needed.

regarding backup error. it's automatic backup before upgrade but if user do full backup manually it's not needed. You can igrore.