Wifi problems on Macbook since installing update

Question

My Macbook can no longer connect to my wireless network at home ever since I installed a software update (NOT Snow Leopard!) at the weekend. It worked fine until I installed the automatic update, which included an Airport Update & Security Update.
It now recognises the wireless network and asks for the WPA password but each time it tries to connect I get a "connection timeout" message. The password is definitely correct and as far as I know I haven't changed any settings.
It doesn't seem to be able to get an IP address, and creates a self-assigned IP.
All other computers in the house (PCs) and 3 iPhones are having no problems connecting to the wireless network, and the Macbook works when connected via ethernet cable.
I have tried the following to resolve the problem
1. Restart router
2. Renew DHCP (doesn't work)
3. Pretty much everything else that you can do through Network Diagnostics/System Preferences
4. Reset PRAM
5. Reset all network settings, deleted existing networks and re-added them.
It's definitely NOT a hardware problem as the wi-fi worked fine on my friends non-secure network and in the Apple store.
I really want to figure out a way to undo this upgrade, but the Apple Genius at the store said that there was no need (annoyingly everything worked fine in store).
My router is a Belkin 54g which is set up through my boyfriend's computer.
If anyone can help I would be eternally grateful, I can't get any of my work done from home unless I am sitting right next to the router as our ethernet cable is only a couple of feet long!
I have seen lots of other posts on a similar topic but none that have provided me with a solution yet.

Answer 1

Please test after taking each of the following steps that you haven't already tried. Stop when the problem is resolved. Back up all data before making any changes.
Step 1
Take the applicable steps in this support article. The Wireless Diagnostics program generates a large file of information about your system, which would be used by Apple Engineering in case of a support incident. Don't post the contents here.
Step 2
Disconnect all USB 3 devices. If you don't know which are USB 3, disconnect all USB devices except keyboard and mouse.
Step 3
If you're not using a wireless keyboard or trackpad, disable Bluetooth by selecting Turn Bluetooth Off from the menu with the Bluetooth icon. If you don't have that menu, open the Bluetooth preference pane in System Preferences and check the box marked Show Bluetooth in menu bar. Test. If you find that Wi-Fi works better with Bluetooth disabled, you should use the 5 GHz Wi-Fi band. Your router may not support it; in that case, you need a new router.
Step 4
Open the Network pane in System Preferences and make a note of your settings in the Wi-Fi service. It may be helpful to take screenshots of the various tabs in the preference pane. If the preference pane is locked, unlock it by clicking the padlock icon and entering your administrator password. Delete Wi-Fi from the service list on the left by selecting it and clicking the minus-sign button at the bottom. Then recreate the service by clicking the plus-sign button and following the prompts.
Step 5
Reset the System Management Controller.
Step 6
Reset the PRAM.
Step 7
Launch the Keychain Access application. Search for and delete all AirPort network password items that refer to the network. Make a note of the password first.
Step 8
Make a "Genius" appointment at an Apple Store, or go to another authorized service center.