It may be that there are several wireless networks in your neighbourhood and this is causing interference with your connection. Try changing the channel number that your wireless router uses.
To do this, connect your PC to the router by ethernet cable, NOT WIRELESSLY, and type its IP address of 192.168.2.1 into your web browser. This will open the router's configuration pages. Within the wireless settings there will be one for the Channel number. The default for most routers is usually 6 or 11 so, if already using one of these, try the other. Your PC's wireless adapter will pick up the change automatically, so you don't need to change anything there.
Is your wireless network "secured" using WEP or WPA encryption? If not, you may have "unwanted guests" using your wireless connection illegally and this will slow it down.