Upload speed drops to zero

The speed test on my 2 Windows7 machines (PC and netbook) have recently started to show a fine download speed but when measuring upload speed the dial moves up to 400 and then gradually sinks back to zero over the next couple of minutes. It doesn't register any final upload speed. I can upload files to emails up to roughly 200kb but nothing any larger.
However my old laptop with XP has none of these problems. Hard wiring makes no difference and it's not the router, because the XP machine works fine with the same router that uses the Windows7 PC.
I've uninstalled Adobe Flash and re-installed it for the Windows7 machine. No change.
Same problem whether Firefox or IE.
Grateful for any advice...
Thanks

Is the router a little on the old side? It is rare, but there are some routers that just don't work with Windows 7. I had an old Linksys one in a coworker's home office that needed to be replaced because her computer was upgraded to Windows 7.

Thanks for reply - the router isn't more than 2 years old, and has worked fine until very recently. The upload problem only occurred a couple of weeks ago - it's been fine up to then. I've tried different ports in the router - no change. Very strange.....