I had exactly the same issue on a 2013 15" Macbook Pro Retina with Windows 7 64-Bit Professional on Bootcamp 5.0.5033. Perfect wireless on OSX and an excellent signal showing on Windows but, once connected, it dropped to poor unless I was right beside and adjacent to the router. Even at that, if I the turned the Macbook 90 degrees, the signal went from 4 bars to 1.
Bootcamp had installed driver version 126.96.36.199 for the Broadcom network adaptor in the Macbook. I manually installed an older driver (version 188.8.131.52). This would not connect for me so I undid it by rolling the driver back to the originally installed 184.108.40.206.
My wifi router is a Netopia 2247-62 and I looked up my internet service provider's home page for the router. It said that the security was set at WPA Version 1 & 2.
I checked the Windows 7 security setting for my internet connection - it was set at WPA-Personal. I changed it to WPA2-Personal and immediately got excellent signal strength and range.
So check that the security setting on Windows 7 matches that of your router. If you do not know how to access your router details, you will need to research it or contact your service provider.
To check or change the network security in Windows 7 - click on the wifi symbol on the task bar, click on Open Network and Sharing Center, click on Manage Wireless Networks, click on your network to highlight it, then right click, Properties, select Security tab and select the Security type to match that of your router. You will then have to re-enter your network security key.
Alternatively if you cannot access the router information, you should check for inconsistencies between the network settings (the security setting in particular) in Mac OSX and in Windows. The nature of this problem is such that the Mac OSX wireless settings must be good and should be duplicated on the Windows wireless settings. To view the Mac OSX wireless security settings, click on the wifi symbol at the top of the screen, click on Open Network Preferences, click on Advanced and then on the wifi tab.
On my Mac in OSX , security was reported as WPA/WPA2 Personal. My Windows 7 settings on the Mac had by default on installation set security to WPA Personal giving poor connectivity. Changing the Windows 7 settings to WPA 2 Personal as described above gave excellent connectivity.