InSSIDer is an essential wireless troubleshooting tool that will quickly help you get the best possible performance from your network.
Install the program on a laptop, choose your wireless adapter, and it'll immediately detect and display your network, and any others nearby. Walk around your network area and inSSIDer will show you how your wireless strength varies in different places. Try to locate your PCs in areas with a strong signal and you'll see faster and more reliable connections.
If one PC in particular has poor wireless speeds, then install inSSIDer there, and again it'll display your signal strength at the location. You might then try repositioning your router or access point antennae, especially if they're directional, to see if they can increase the strength of your signal. Moving the entire router may help, too, but be careful you don't improve the performance of this network PC at the expense of the others.
Often the most useful aspect of inSSIDer is the way it detects other networks, that your neighbours may be using. There can be many more than you think, and if these are using the same channels as your network, then you'll be fighting over the available bandwidth and slowing each other down. If there's a strong signal nearby, and it's using the same channel as your network, then change your hardware to use something else (usually channel 1, 6 or 11). You should see an immediate and major improvement in network performance.