Internet is slow and drops out on iMac when another laptop is using network.

I recently purchased a 27" iMac and am having trouble with my internet. I have tried so many solutions but nothing seems to work! I have relocated my router, purchased an Airport Express, moved to location of my mac to another location in my house, but nothing works! The problem occurs when my Mum uses her latop on the network simultaneously. The connection on both computers is extremely slow, and eventually mine cuts out. If her computer is switched off then my iMac has no problems and is great. I have called AppleSupport numerous times and each time they don't fix the problem and I'm beginning to wonder if my Airport Card is faulty as suggested by one Apple employee. I am at breaking point and need a solution to this problem. I would appreciate any help at all! Also, I have numerous other Apple devices such as iPad, iPhone and Macbook (as well as other non-apple products) none of these devices have any issue using the wireless network. The problem seems to be specifically with my Mum's laptop and my iMac. Thanks!

This sounds suspiciously like two machines with identical addresses...

(Check in Network Preferences -> Airport -> Advanced -> TCP/IP on both machines)

Or possibly, if you use your laptop elsewhere, on a system behind a proxy, you'll have to create a new 'home' profile without the proxy.

(Network Preferences -> Airport -> Advanced -> Proxys)

Perhaps...

Along the same lines as darkslide.....sounds like an IP address clash.

Check your router ro make sure it's allocating different IPs to each device and check your Mum's laptop to make sure it hasn't got a static IP set.

If you don't know how tocheck any of those things post back with your router make and model and the version of Windows used on the laptop