iPhone 4 not finding my Windows 7 ad-hoc wireless network when it restarts

I have set up an ad-hoc wireless network on my Windows 7 machine. My iPhone 4 connects with no problems (except it only allows WEP security, not WPA2) and I can share the internet connection. However when I restart my Windows 7 machine, the network still appears in Windows 7 but my iPhone 4 can't find it. I've made sure the network is permanent (I realise ad-hoc networks are designed to be temporary but Windows 7 remembers it) and I have to recreate the network every time I want to use it.

Any help here?