IMac (early 2009) not connecting to wifi

Question

Excuse my typing I'm on my iPad so there may be errors!
Day before yesterday my router dropped the signal. By morning it was back on all all devices (iPad, iPod, Xbox, my dads pc etc) were working fine, but now my iMac will to connect to the wifi. It's driving me Barmy! I've called the provider, there is no problem their end any more, and they know nothing about macs so couldn't help.
I have tried everything. I've renewed my dchp lease, I've restarted and reset the router, restarted my mac, unplugged everything, forgot my network, created a new location... I've been trawling the Internet on my iPad for hours at a time looking for a solution, but there is nothing. Every single problem I've encountered seems to have a different cause, or their computer is different etc.
I am very, very reluctant to mess with my system settings so please don't suggest I go in and edit system files because I'd mess it up!
I am using 10.7.5 and my iMac is early 2009 20". I am with virgin media and cannot connect via Ethernet due to where the modem is placed. Is nearby, but plugging it in would mean wires trailing across the top of the stairs which would be dangerous!
Cn anyone help? I tearing my hair out! Tank you!

Answer 1

Some users with various Wi-Fi connection problems have reported that they were given the following instructions by Apple Support, sometimes with success. I can't vouch for this procedure myself. After completing the procedure, you'll have to recreate all your settings in the Network preference pane. Make sure you know how to do that before you begin. Taking screenshots of the preference pane may be helpful.
Step 1
Back up all data.
Triple-click the line below the line you're reading now to select it:
/Library/Preferences/SystemConfiguration
Right-click or control-click the highlighted line and select
Services ▹ Reveal
from the contextual menu.* A folder should open with an item named "SystemConfiguration" selected. Move the selected item to the Trash. You may be prompted for your administrator password.
*If you don't see the contextual menu item, copy the selected text to the Clipboard (command-C). In the Finder, select
Go ▹ Go to Folder...
from the menu bar, paste into the box that opens (command-V). You won't see what you pasted because a line break is included. Press return.
Step 2
Reset the System Management Controller