this is a difficult one to answer as there are so many variables to take into account.
most common things to check would be...
interferance... anything that might interfere with the signal ie.mobile phones, digital phones,metal objects (tower case).
also make sure the router is passing an i.p.address to the pc ...sometimes the connection will sit for ages trying to get the ip address from the router .
and will appear to be connected ,but is just trying to get the i.p. from the router .
I.Explorer make sure the lan settings in connections are set to auto detect settings .Dave