To do this from the command line is actually very easy. First, open an instance of vim. Then, in a new cmd, type:
vim --remote-silent +e filename
You could also do
vim --remote-silent +split filename
vim --remote-silent +tabedit filename
Depending on the behavior you want.
However, running stuff in the command line isn't always the most convenient, especially in windows. To add this to the context menu, e.g. right click a file then have this option, do the following:
Make a file with a
.reg extension, and save this to it:
Windows Registry Editor Version 5.00
@="Edit in existing vim"
@="\"C:\\Program Files (x86)\\Vim\\vim74\\vim.exe\" --remote-tab-silent +edit %1"
Double click this file. This will add "Edit in existing vim" to your context menu. Now, you can right click on a file, and it will open in a new vim if you don't have one open, or in a new tab on an existing vim if you don't.
If you don't want this in a new tab, just change