WLAN sharing

Hi guys

I have had this issue with my set up for WLAN for some time.

The Apple support and my internet provider have so far not been able to resolve my issue.

I am a secondary user of an Arcor Easy Box A600 WLAN. The primary user is an MSI windows vista laptop.

I have an old Toshiba laptop that is also set up as a secondary user.

The mac mini is configured correctly, and I have restricted it to 256 k/bits with waterroof.

However when I use the mac mini airport alone it is sporodic but workable.

Likewise when in a shared WLAN situation it slows all the traffic and slows the whole system down both for the laptop and the mac mini.

Any ideas how to improve my connectivity in general with the mac mini?
Also when sharing the WLAN?

The only solutions I can come up with is

a) doing some home redecoration (DIY), linking up the mac by ethernet, and using the laptops as WLAN.

b) investing in an aiport extreme base station.

The latter option I am not even sure would resolve the problems.

Any ideas would be great?

cheers stew.t.