Only one computer on the Internet. Why?

Hi. I've been having loads of problems with the Internet and for some reason can only get it on one computer. Sorry if it gets confusing.

I used to have a network set up, which involved two computers and sometimes a laptop. It was a wireless network, but also worked wired. But now the main computer can not be used and so I tried setting up the router on my other computer. Now this is Vista which is possibly why it won't work (the other computer was XP), but it was part of the network before and the router seems to be sending out a signal (my PSP goes online with it fine) yet my computer won't even though it is wired to it and shows it is.

I have tried to get my laptop (XP) back online - no settings have been changed and it all seems fine yet even with a direct link from the modem to the laptop through cable it refuses to work. It used to work fine like this a few months ago.

My sister just bought a brand new laptop (Vista) and this can not get online wirelessly through the router. I tried an old router which is just wired and my computer was online yet hers wasn't. I can't set up any connection for it except wireless it seems.

I also have an older computer (Windows 2000) and this too would not go online, even though it used too.

So basically I have one computer directly linked by cable to the modem that will go online and that is all, despite seemingly having a working router, all settings correct, everything installed properly and knowing I have the Internet as I can get it on here.

I don't understand what the problem is. I've tried a number of ways to solve it but have yet to get anything to work. So is there anyway I can get the Internet back on all the computers? Any help or advice would be great!

Thanks

BleepingComputer.com: Only one computer on the Internet. Why?