How can I set up a WEP wifi network hosted by my PC running Windows 8, which draws its internet connection from wifi?

Question

My Nintendo 3DS can connect to WPA networks, but not when running the old DS games - there it requires an open network or WEP encryption at the most. My new router doesn't support multiple SSID's unless I flash it with DD-WRT, and I don't want to do that in case it bricks the router - I asked on the DD-WRT forum, and that's apparently a remote possibility with the one I'm using! I don't want to set my main network to WEP, because that would disable 802.11n and slow the network down to 802.11g speeds.

So I'm trying to set up my PC as a wireless access point for the DS to use, but apparently Windows 8 doesn't support ad-hoc networks anymore. I tried the hostednetwork feature of netsh, but that only supports WPA, so I can't use it. I also tried Connectify, but that won't let me share my wifi connection over wifi.

Am I stuck here and need to set up the old router as a secondary AP? I tried that once but had all sorts of IP address conflicts. I think it was because I didn't understand subnet masks, so I could probably do it right this time, but I'd have to dig the old router out of wherever I put it - and it was kind of flaky to begin with, not wanting to load certain websites, so I'm not sure if it would even work anymore for Nintendo WFC!