Modifying The Path Variable
Open a new window in Windows Explorer and navigate to the folder you installed Ghostscript to. (By default, this should be "C:/Program Files/gs", but your setup may vary.) Go into the "bin" folder of your Ghostscript install directory (you may have to go into a subfolder with the Ghostscript version number, such as "8.70") and copy the folder location from the address bar by selecting it and pressing Ctrl-C. (If you don't have address bars activated, you can just write it down, but make sure you copy it exactly).
Now go back to the "Edit System Variable" dialog box and paste the text at the end of the list of "Variable Values" in the bottom text entry box. Each value needs to be separated by a semi-colon, so make sure there's one between the last entry and the one you just pasted. Be careful not to overwrite any other values. If this happens, press "Cancel" to close the dialogue box, then re-open and re-paste the location of the Ghostscript "bin" folder.
Repeat this process for the "lib" folder contained within the Ghostscript install directory, again making sure that there is a semi-colon separating the items in the "Variable Value" box. Once this is done, click "OK" to accept the changes you made.
Now when you run Inkscape, you will be able to see EPS files in the "Open File..." dialogue, just as you would with SVG and other Inkscape-readable files. Select an EPS file and open it.
Inkscape will now run through the EPS import script by opening the Ghostscript interpreter. When the process is complete, a dialog box will open with some import options. Click "OK" and your EPS file will be open for editing.