What can you suggest is causing our housemate's laptop to stop the rest of our internets working?

Basically our internet has been acting up lately, and being ridiculously slow. However I heavily suspect it is because of our housemate. Reasons are: it seems to only act up when she is on the laptop such as she went away for the weekend and the internet worked fine, then as soon as she returned and went upstairs it started not working again.

So in your experiences, what may be causing this problem so we can follow up on it?

We use a netgear (it was free, i’ve had this router before and its crap but it was working fine when we got it so I shrugged it off) wireless router and are on Virgin Media Broadband.

Thanks