Routing all Traffic through OpenVPN Tunnel

Set the VPN Interface as your def gateway (making sure it's up)...

In Linux this would be something along the lines of (of a.b.c.d is the address of your VPN interface):

sudo ip route replace 0.0.0.0/0 via a.b.c.d

Or you can use "/sbin/route" but the syntax is slightly different.

If you want this to persist through reboots you can add it to /etc/rc.local but you probably want to use the specific method for your distribution. I see you are using Arch Linux so the file you want to edit for a permanent static route added at boot is /etc/rc.conf. Check here for more info.

Good Luck!

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I just wrote this answer and then re-read the question....

For Windows you want to do this (on a command line):

route add 0.0.0.0 mask 0.0.0.0 a.b.c.d

or possibly:

route change 0.0.0.0 mask 0.0.0.0 a.b.c.d

On Windows Vista and subsequent, it is often necessary to tell OpenVPN:

route-method exe
route-delay 2

Otherwise the routes cannot be set.

Then I recommend not to change your routes manually but to use the OpenVPN dedicated setting:

redirect-gateway def1

There is a big difference between the two: your route interferes with the default one and when your local DHCP renews your lease or something, it might restore the original default route and mess things up. The redirect-gateway def1 rather installs two routes: 0.0.0.0/1 and 127.0.0.0/1. It leaves the default route, is more specific so has precedence over the default and is easily removed.