There are a number of devices which use the same frequency band are wi-fi and it could be any of these.
One you haven't mentioned is Digisender, a device which transmits a TV signal from one room to another.
You say that it was no better when you used an ethernet cable to connect, which might suggest that it isn't your wireless connection at fault. In any event, if you double-click on your wireless network icon in the Taskbar it tells you the wireless speed (effectively the signal strength) you are getting.
Do you ever check your internet download speed? Try going here click here and clicking on Speed Test. This tells you how fast data will transfer from the internet to your pc. It could be that your adsl connection is having a bad wire day.
The other check that you could do is go into your router's setup pages. One of them (Status, ADSL or some similar name) will tell you things like the Downstream Rate, Upstream Rate, Downstream Margin, Signal/Noise Ratio and a load of other guff. The interesting figures are Downstream rate or speed (in Kbits/sec) and downstream margin (S/N ratio) in dB or sometimes no units). The downstream rate is the connection speed of your modem - the higher the better - and the S/N ratio tells you how noticeable the adsl signal is over the background noise - again, the higher the better. You can't directly change these in the modem.
Also, make sure that every phone etc socket has a microfilter and that nobody has removed one, dislodged one or started using a new or previously unused extension.
Hope you get it sorted soon.