Wireless connection lags on PS3 -- other computer using too much bandwidth

Hi.

I have a DSL internet connection. Upstairs my computer (Dell inspiron 600m) is connected via Ethernet wire to the modem/router. -- Downstairs I have a PS3 connected wirelessly. Whenever one streams video, for example, on the computer upstairs, the connection downstairs slows and lags, making online play difficult. I've been told that this may be because something on my computer is using too much bandwidth. -- I started my computer on safe mode with networking, watched some videos on youtube and the connection to the ps3 still lagged. How can I fix this? that is, be able to use youtube upstairs without causing lag on the ps3 downstairs?

Thank you for taking your time to read this and for the help you provide.

John