When I log into windows, I automatically have a new DHCP lease from my ISP, and can log in normally (I have a broadband connection). But when I have logged into windows and decides to switch to Linux, I need to wait for the lease to expire to be able to log in again (usually about 30-40 minutes).

Eg. When I start Linux, Eth0 responds [FAILED], but if I wait 30-40 minutes and tries to get online using $ IFUP Eth0, it works fine.
Any ideas how to renew the dhcp lease at startup when switching O/S?

Before logging off Windows have you tried to release it manually, then boot into Linux and see if you attain a new IP at bootup ?

Sounds like its more of a problem with your ISP than with your machine though.

nope, haven't tried that..

just thought there was another way, cause most of the times you don't remember to release it, and you're getting used to windows doing it for you

I have the same problem with my ISP. I contacted them about this and they said my lease is 10 hours ! I had tried everything: manually releasing IP in windows, ifconfig eth0 down, ifconfig eth0 up, I reset my cable modem (which, BTW, if you switch from you win box to your Linux box, you'll have to reset the modem as it's sending to a different NIC), etc.

My ISP told me that if I need the IP released earlier than the 10 hour lease time that I just give them a call and ask them to release the IP and the MAC address of my cable modem. I would then unplug the power to my cable modem for a few seconds, hook up my ethernet cable to my NIC, power on the modem, and all is well.

However, I've alleviated that problem by never putting my wife's Winbox online. She was desperate to get online one night, so I walked her through logging in from command line and startx. She'd never used Linux before, and had never seen Opera. During my 9 months of running Linux exclusively on my machine, she never wanted to use it or even see it. She now has a user account and thinks that Tux is the cutest thing to grace the planet (besides me of course ). She played around in my user account for 5 hours that night and is in love with the Gimp. In fact, she hasn't used her Winbox since I introduced her to Linux 2.5 weeks ago.

Sorry, that was a bit off topic, but I find it an amusing anecdote to my solution to the IP lease problem.