Wireless connects, but browsing non-existent

I was using wireless network connection on my Dell Inspiron 6400 laptop for months without any problems. Suddenly, three days ago I started having a problem - my computer still sees and connects to all wireless networks, but the Internet browsing is extremely slow. My browser (I.E.8, Firefox and others)would see the web page, but it takes forever to download it. Most of the time the I.E.8 simply gives up and says that it can not connect to this web page. My computer has Dell LAN wireless card, so I tried to use different external USB wireless card - the same result. The speed of the network connection is 10 Mb, so it should not a problem. I tried for two days different solutions - checked my LAN settings, disabled the antivirus software etc. Long story short, I have found that the only thing that works for me is to restart the Network Store Interface Service. Once I restart it, everything returns to normal and browsing is fast again... untill I restart my computer or wake it up after sleep. Then everything is wrong again with impossible Internet browsing. I have to restart this service again and again. I need help on how to resolve this issue permanently. Constantly restarting wireless services is not a solution. Need to mention that I was not installing any programs or changing any settings at the time when the problem occurred. The only event that coincided with that was the installation of the last updates for Windows Vista SP2. I tried to uninstall the .NET Microsoft 4 Client, but it didn't help. So, I installed it again. I hope you can give me a usefull suggestion on how to proceed further. Thank you.