If you do not have security set on, chances are that someone else nearby (1 - 2 houses away) is hitching a ride on your wireless network and stealing all your bandwidth. Which will cause the symptoms you have described.
Set security up on your Router (see other threads in this forum as to how) a.s.a.p.
Even if this still reoccurs, you have eliminated the major loophole for all wireless networks. The next common culprits are devices which operate on or near the wireless network's frequencey (2.4Ghz). Common devices are cordless DECT phones and base stations too close to the wireless router or PC wireless cards/aerials. And microwave ovens.
The above assumes that neither the router nor wireless client PCs are moved around, nor some metal obstacles are moved to block the radio transmissions between the router and client PCs.