Strange wi-fi Windows 7 connection problem

[singlecoil]

We have four computers at various places in the house all of which access the internet through a BT Home Hub. All are running W7. For some reason the newest and most expensive of them (more than the other three put together) has decided to stop automatically connecting to the hub. And the hub doesn't even show in the list of available connections, although my three of my neighbours' routers do. Then every now and then it will appear. Then it appears to connect, but not properly.

When it's doing this it shows as connected in the list of connections available, signal strength very good, but no internet access. Except very occasionally, when it appears to be properly connected with 'internet access' but the pages load really slowly, while the computer next to it is banging them out in almost no time.

Windows trouble shooter is as useful as it usually is, I've tried resetting the router, all other computers still working fine.

Any ideas as to what I could try next?

Thanks in anticipation.

[singlecoil]

I should have said, it's a desktop with a wi-fi card. It's since connected to the internet but is ridiculously slow (most of the time, with occasional bursts of normal speed). According to Windows the driver for the card is up to date. This problem is recent, up til a short while ago it was working perfectly, and I'm not aware of anything else having changed.

[singlecoil]

I've still got a problem.

The wi-fi signal is good, it has external antennae which I've tried in various positions but in all of them it shows 4 bars as opposed to the working perfectly W7 computer next to it which has an internal aerial and shows 2 bars.

But it's as if the signal, once it's inside the computer, is being seriously degraded. There's a little bit of internet that gets through, but a YouTube video that streams easily at 480p on the other computer continually buffers at 240p on the one with the problem.

Anything else I can try?