Reluctant wireless connection

Hi

I have an Inspiron 1545 which is connected wirelessly to a Linksys WAG160N gateway. My wife and I each have desktop pcs which have wired connections via a switch to the Linksys and these both work fine.  The problem lies with the laptop.  When this is powered up, there is no identifiable internet connection and the small symbol in the notification area has a yellow triangle over it.  If I go downstairs and switch off the Linksys and disconnect power and telephone cables for 10 secs then reconnect, a wireless connection is then established with the laptop.  The laptop is not used every day, but the next time it is switched on, the same scenario occurs.  I am hoping that the remedy is a simple adjustment of settings, but I cannot see how todo this. Please can you help?

Thank you.    Peter:emotion-1: