Internet wont work unless I reboot computer

I have Windows XP Cable internet and use Firefox. Lately if I logoff of screen and log back on , I can't get on the internet unless I restart the computer. Any suggestions? Thanks in advance.

 

Do you have a firewall? If so, what is it?

Also, you've said you use cable, do you use any form of connection software? If so, what is the cable service and/or connection mechanism?

I've had a similar problem but never realy ascertained what the cause is but I think, in my case, that it is the Firewall (Norton Internet Security) or the connection software that doesn't fully reinitialise when the account is logged out and back in again.