After claiming that the USB wireless network dongle (a Belkin F5D5070) was working ok on Emma's computer, it started playing up again. It would still lose network connection a few times per day and require re-starting. At least it didn't crash the computer forcing a hard-reset (unlike on my machine). We tried all the different hints and tips off the Internet:

• Let the manufacturers software handle the network
• Let Windows handle the network
• Enable encryption

etc. etc. Nothing helped.
We took it back to the shop today. The person on the service desk was less than helpful. We explained that it seemed to work ok but would lose network connection several times per day while other computers in the same room worked fine. They took it into the back room to test, then came back 2 minutes later claiming it worked ok, completely ignoring our insistance that it only worked intermittantly. They suggested we phoned the manufacturers, and also gave us a number for their support line. I asked how much the call would cost and whether it was premium rate. I was ignored and simply told again that I should phone the number.
Back home, we phoned Belkin. They blamed Compaq and suggested I updated my USB drivers, despite me having a brand new machine, almost certainly having newer drivers than anything Belkin had when they developed the thing. Next, we phoned the support number the shop gave us (we checked first that it wasn't premium rate). The guy at the other end was a lot more helpful. He said that although he can't really offer support because it didn't come pre-installed in the computer, he'd had personal experience of the device and said, off the record, that he thought it was a heap of crap. He gave us a code to quote which meant we were returning it as faulty. We also mentioned that the original salesman told us that opening the case to install a card would invalidate the warranty, and that was the only reason we bought the USB version. He told us that wasn't strictly true and the warranty would only be invalid if we'd damaged the computer in the process.
We drove back to the shop again and spoke to a different service person this time (after waiting in a queue - we must have chosen a busy period for complaints and returns). We explained what had happened over the phone, that we only bought the USB version because we'd been misled about the warranty, and quoted the code we were given. Finally, we had a result: after disappearing for a minute to consult someone else, he returned and said we could exchange it for a proper internal card.