Also, let us know what types of comps you have, and how they're connected. Are you using CAT5 cables, a hub, or a switch? Hubs always reduce your connection speed if you have two or more computers connected to it.
Lastly your connection type and provider make a huge difference.
If you're even using AOL DSL service, you'll still lag. The AIM.exe file was ranked as one of the laggiest pieces of software the casual user will ever use. It's a pain in the ass to have to work around with it hogging up taskbar and system tray space.
If you're using any type of AOL, that's your problem. If you're on dialup and your friends are on DSL, the latency difference is too great for them to join, try letting them make the game.
If you could just tell me the following information, I could try to figure out your problem for ya
Computer Brand:
CPU (Brand and speed):
Amount of RAM:
Hard Drive Used Space/Total Space:
Connection Type:
Operating System:
Typical Programs Running While on StarCraft:
If you're unsure of any of those categories, I've provided a sample of my setup below...
Computer Brand: Hewlett Packard Pavilion ze5000s Laptop
CPU (Brand and speed): Intel Pentium 4; 1.8gHz
Amount of RAM: 512MB
Hard Drive Used Space/Total Space: 8.4GB/40GB
Connection Type: Verizon Online DSL connected to 10/100 Etherfast Ethernet NIC
Operating System: Windows XP Pro
Typical Programs Running While on StarCraft: UltimateBot, BG3, Winamp, AIM, LeapFTP, BlackICE
Kazaa is a major factor in game lag as well. If you have that open, make sure you close it in your System Tray (it's a blue and yellow icon on the right-hand side of your taskbar) by right-clicking on it, and going to "Close Kazaa Media Desktop". Simply hitting the 'X' button at the top right of the Kazaa window does not close it, or the active connections. Many people make that mistake.