Usually well over £40 but just spotted it on Amazon for £24.95.
High-speed Wireless-N (draft 802.11n) networking for notebook and desktop computers
MIMO technology uses multiple radios per band to create robust signals for maximum range and speed, with reduced dead spots
Operates in the 2.4 and 5GHz radio bandschoose the one with least congestion in your area
Much faster than Wireless-G, but also connects to Wireless-G, -B, and -A networks
Performance investment protection: compatibility with Wireless-B (802.11b) and Wireless‑G (802.11g) standards
Supports up to 256-bit encryption (WEP, WPA and WPA2)
USB 2.0 interface
Easy-to-use setup wizard
Compatible with Microsoft Windows 2000, XP, and Vista
This is the cheapest Dual Band adapter I have seen and will go nicely with my router in 5ghz mode, zero interference, faster than 2.4ghz and no-one else in the local area uses 5ghz so harder to hack into.
For info, only some routers have a 5ghz mode so you will have to use the 2.4ghz mode if you haven't got one. The best thing about this is that more routers will be released with 5ghz in the future and you will not have to buy another adaptor to take advantage.