Wireless internet problem!

So heres the situation. We have one desktop PC downstairs which is connected to the cable connection. Along with that we have a linksys wireless router. The PCs that are connected to this wireless router are a desktop and a laptop computer. Now whenever I turn on the desktop downstairs and noone else is online in the house, the internet is extremely fast. But when someone else is online (for example the desktop upstairs wirelessly connected to the internet), the internet for both computers slows to barely a crawl. My question is how would I get 2 or maybe even three PCs online at the same time without the network slowing down, if possible at all?