I have and airport express and my IMac works fine with the internet connection, but my husband's PC in the basement has problems. The wireless connection comes and goes. One minute it says good connection the next unavailable. I have a PC laptop that will pick up the connection in the basement just fine.
Simply put, every wireless device/aerial is different and the connection strength is always just approximate.
You can either try changing the wireless adapter, buying a bigger/better aerial, or if this computer is a desktop and always in the same place, look at using RJ45/Ethernet as it is much better when compared to wireless in everything (Other than portability if you want to go around the house!).