Can't turn on Network Discovery because of services problems

Question

Windows 7 SP1 Home Premium 64 bit with all the updates. Wireless network connection.

I can't connect this PC to my NAS; none of the other PCs on the network, or the NAS, appear when I browse the network. This is a home setup, I have checked that all devices are in the same workgroup. I am not using Home Groups. There are no problems browsing the internet or printing to my HP Photosmart Wireless printer.

It turns out that Network discovery is turned off, but if I turn it on it immediately turns itself off again.

So far I have found that:

• Network Discovery won't turn on because the UPnP Device Host service is stopped.
• The UPnP Device host service is stopped and won't start because it depends on the SSDP Discovery service and the HTTP service. The SSDP Discovery service is stopped.
• The SSDP Discovery service is stopped and won't start, it depends on the HTTP service.
• There appears to be no such thing as a HTTP service. It does not appear on the Computer Management | Services page (I've checked that this is normal, there is no HTTP service on another, working, PC.)

That's it; I have no idea what to do now. Do you have any suggestions on how to fix this? Thanks.