Did you manage to solve this?
In case it's helpful, I've found a partial solution.
The utility WirelessMon - http://www.passmark.com/products/wirelessmonitor.htm
It allows you to override Windows wifi selection, and explicitly connect to an AP by MAC address.
I had the highly frustrating problem of needing to use a public hotspot network with a portal website, and every time my wifi roaming jumped AP, every ~5 minutes or so owing to two of them being very similar signal strength, I'd have to re-enter my login details.
The downsides of using WirelessMon are, it's only a 30-day trial, and it also tends to screw up the connection itself by constantly scanning in the background. The best solution I've found is to set its sampling rate to the maximum of 60 seconds, which helps enormously compared to the default of 3.
If anyone knows of a free app that does a similar job, please post about it!