Unless there is a problem with your wireless card, the speed shouldn't be effected by the computer. However, it can be effected by how many devices are connected to the WiFi (and what they are doing), the ISP having issues (which is so common) and also the modem/router. And the location of the modem/router can be effecting if there is anything interfering with the WiFi signal.
The first thing I would try would be to reset the modem/router. Then test the speed once it is fully reloaded. Also, the more devices using it the worse your connection will be. And if any of them are graphic intensive, like say playing an online game from a laptop or having a gaming consol that you are going online and playing, (as an example) would seriously use up your connection.