AirPort Express and Bt1800HG problems - please help !

Equipment - PC with Windows XP Pro.
BT 1800 HG Wireless Broadband Router
AirPort Express Base Station
imac G5 with AirPort Extreme Card

My Situation: I have a wireless network set up with the PC, BT Router and Airport Express connected to it - this however is not an Airport network. I tried to connect my imac to this network but was unsuccessful e.g. the green light on the airport Express was on but my imac could not detect it whatsoever. (antenna was properly connected). Any ways to remedy this so that i can connect wirelessly to the 1800 HG through the Airport express ?

I also have been wondering if something is wrong with my airport.
I have a macbook with 10.5.5. At home I have an airport express & at work I have a bell canada wireless router.
Some times when I'm at home my computer connects to another unsecured router. When I try to connect to mine "sometimes" it asks for the password, witch already shows up. I hit connect and get a "connection time out" this will go on and on until I unplug and replug the airport express.
Also sometimes when I am connected my computer goes so slow it's stupid witch is fixed by unplugging & replugging the airport unit. I don't think it's the air port because my Iphone works perfectly while this goes on.

At work with the bell router my computer works seamlessly. But my brothers macbook pro 10.5.5 gets kicked off all the time and has to reboot his Wi Fi by shutting the airport off then on. I have never had a problem with my iPhone Wi Fi witch leads me to believe there is a software problem somewhere in 10.5.5 or my brother and I have picked up a bug somewhere.

Is anyone else having this problem or just have an idea about what might be wrong, let me know.

Thanks,
Brad