Windows 7: Slow wireless internet on one specific windows 7 machine

Hi all. I've looked through the various forums for my specific problem and haven't been able to find anything useful. My problem is that my new build (about 2 months old) is having slower and poorer wireless connection to the router. It has gotten to the stage where connection is very intermittent, and when connected can take 5 minutes to open an email. The specifics are;
the problem machine is entirely new; no component has been used before in another machine. All drivers are current. Virus and Malware checks are clear. I am operating Windows 7 Professional 64 bit.
It is not the router; there are 3 computers being served by the one router, and only this machine is having trouble. The other 2 machines are laptops, one running Vista and my laptop which is running Windows 7 Home 64 bit.
It is not reception; the problem desktop gets as good or better reception than my laptop when placed side by side, yet just this afternoon while I was looking through these forum pages it was taking so long to open a page that I booted my laptop, went through the whole start up, logged on to this forum and opened three pages before the desktop had completed opening a single page.
I uninstalled the drivers for the wireless card (ASUS card) and reinstalled them. It worked great for 2 hours, then disconnected again.
Sometimes data transfer is very good, just lately the 'good' times amount to maybe half an hour a day; the rest is just frustration.
Any thoughts?

It could be an antenna issue. Most wireless cards have there antennae fixed on the back, and this can lead to intermittent problems simply because there is a large mass of grounded metal (the computer case) in close proximity. The solution is to fit an extension cable and mount the antenna away from the case. Even on top of it is much better than behind it, as the shielding effect of the case will be greatly reduced by this simple measure.

Thanks for the suggestion. Not discounting it, but the actual signal strength is better through the problem desktop than the laptop, it's the data transfer rates that are a problem. In other words, although the desktop shows 5 bars reception and the laptop only three, the laptop is much faster at actually opening pages.
The desktop is in a bad location; behind a wall, in a corner under a desk with metal legs, yet despite this shows a strong reception. It may be possible that the data is being distorted somewhat despite the strong signals, so I'll try different antenna arrangements first and see if that helps.