Only seem to have these problems around the evenings to nights
When others around you log into their routers! Download Inssider from the link below and check what channels are being used in your neighbourhood. Choose a channel as far from those in use as possible.

On the chance that it is Vista playing up, download and use Vista Anti-lag, this stops Vista looking for other networks, dropping yours as it does.

I am assuming that you have Vista as it is a new PC.