Receiving a file over ethernet very slow

I have 5 computers connected to my router on a gigabit LAN. Two of the computers are WINDOWS 7 machines, one is a Windows Vista Ultimate machine and the other two are Linux. Everything works fine and always has worked fine until yesterday.

On one of Windows 7 machines I can copy a very large file from the machine onto other machines and get 27 Mb/s transfer speed. On the other hand, transfering a file from any other machine (Windows 7, Vista, Linux) on to this "broken" computer I only get up to 10 Kb/s. So, for example, transferring a 500 mb .avi file onto the broken machine will show as requiring 3 days to complete.

Here is the perplexing thing... running Fedora 14 in a virtualbox (with a bridged network adapter) on this broken computer allows me to transfer a file onto this broken machine at normal speeds. So, the exact same 500 MB .avi file that Windows 7 says will take 3 days to transfer will completely transfer over the same network with the same hardware onto the same machine within seconds. Also, this machine dual boots into Linux and everything works fine under Linux...

I tried updating the driver with Hardware Manager in Windows 7... but it says that I already have the latest driver.

I don't know where to start with troubleshooting this as it seems pretty obvious that it is somethjing wrong at the level of Windows 7 on this machine and I do not have this problem at all on other machines.

Suggestions?

What additional information would you require? Incidentally, I disabled the firewall and the transfer speed went up to about 1 MB per second, which is still very slow but faster than before.

It works fine with Linux so it is not a hardware problem but... here goes:

The motherboard is a BIOSTAR TA790GXBE with a Realtek RTL8111DL network adapter built into it.

Thanks for the suggestion.

Actually, I uninstalled Windows Security Essentials and it works fine now. I have no idea why that would be the case but for the last couple of days there would be these seemingly windows endorsed advertisements that would pop up from the little flag in the notification area by the clock and it seemed to have started when I installed "Windows Security Essentials" the other day... so I just uninstalled that.