I had same problems for months...
I'm using lenovo x201 with router, issues are quite similar to the previous ppl's, I can use my iphone and the other laptop with vista to access the wireless, but not win7 for the lenovo one. I went thru a lot of websites asking the problem, so far, I found yours seems to be the only one solved my problem. I don't have 802.11g though, two options, one 20m sth, the other is auto, I changed the setting from 20M one to auto, the speed immediately increased to 150M hz from very slow speed (which shouldn't be the case).
thank you so much and will let you know if if keeps working fine.
After some additional research, I have found others having similar problems with that particular card which evidently uses a Broadcom chipset.
Evidently the driver (new versions) default to setting the "BSS Mode" to 802.11n and some have found that changing that setting to 802.11g has resolved their issues. So....
Go back into device manager
expand network adapters
select then right click the problem wireless adapter and choose properties
select the "Advanced" tab at the top
In the left column titled "Property:" see if you have a BSS Mode listed. If so select it and in the right column titled "Value:" use the drop down arrow to change from 802.11n to 802.11g
OK your way back out of there and reboot and see if that helps.