Looking for help with connection speed

Hello everyone, I have a problem and am looking for help with my desktop wireless speed. I apologize in advance if I get terms wrong or am not very clear with my problem, I know next to nothing about wireless networking or their components. I did a google search on my problem to see if I could fix it on my own but no luck.

We have 3 computers, 1 hooked up with an ethernet cable to a netgear cable modem and one with a Belkin N1 wireless USB adapter, they both run fine but the third computer is using a Linksys wireless g USB network adapter and the speed fluxuates from 18mbps to 48mbps and signal strength fluxuates from low to very good. When I'm browsing the internet it seems to be slower than dial up, pages will not load up and sometimes will error out. The link light on my network adapter will stay off for long periods of time and then start flashing for a few seconds and then shut off again and repeat.

From what I've read about distance and interference for wireless signals, the other wireless computer that runs fine is the same distance and doesn't have any more or less interference than the other one that I can tell. Also all networking components are comcast, save for the Belkin USB adapter, they came out and installed everything but are really no help in helping me to solve this issue.

I would appreciate any advice or tips. Thanks in advance for the help.