I know this post is quite old but I have had a very similar problem to Duckula and it has taken me 3 days of trial and error to get my wireless connection up and running. But thankfully I have a solution (at least for my system) and I'm hoping it will help others having the same issue.
I bought the TP-Link TL-WN350GD for my system (ASUS P8P67LE, Intel Core i5-2400 CPU 3.1GHz, 4GB Corsair RAM, running Win 7 Ultimate 64-bit SP1), installed it into the PCI slot, and installed drivers (either from the TP-Link CD or using Atheros AR5007G) but it could not detect any wireless networks.
Just like Duckula, my network adapter was appearing in Device Manager and apparently 'this device is working properly', but no wireless networks could be found. Manually attempting to install a wireless network returned 'an unexpected error occurred' with no fix available. Using the Win7 troubleshooter suggested a problem with the driver but this could not be repaired, with numerous uninstall/reinstall attempts of different drivers.
I had my laptop right next to me and it was connected fine to the wireless network. My partner's desktop computer has exactly the same hardware as mine and was also connecting fine - so I swapped over our wireless cards (I still couldn't connect) and our HDDs (still no connection). This indicated a software problem on my computer.
I then updated the BIOS and changed some other settings as recommended by other forum posters (e.g. enable PLL overvoltage), but I still couldn't find any wireless networks. Then I used CCleaner to clean up the registry - still didn't work. Of course I also disabled and re-enabled the network card - still didn't work... We even bought a new router (Netgear N300 DGN2200)!
Reinstalling the OS was going to be my next option until I had a closer look at my wireless adapter properties (Network and Sharing Centre/Change Adapter Settings/Right click on Properties of wireless card). It turns out that Trend Microsystems had a driver associated with my wireless network adapter. I uninstalled TM ages ago but it was a difficult uninstall and on closer inspection, many drivers and registry entries remained. I unchecked this box, removed all TM registry entries (using the guides here:
How to Uninstall the Trend Micro Client/Server Security Agent Manually | eHow.com and
Manually uninstalling the Security Agent - Worry-Free Business Security), and removed all TM drivers from my System 32 drivers folder, rebooted and immediately I was able to connect to my wireless network. Hurrah!
So... long story short, if you're having a similar problem to mine, check your antivirus settings and firewalls as it could be a simple fix.