does anybody know what can I do to solve it?

Everytime I try to boot ubuntu, it doesn't even show that screen where you choose the kernel, it just go back to the post screen

I use ubuntu 10.4 installed via wubi in a dual boot system with Windows 7.
And I have a Dell Inspiron 1545.

I don't use Wubi. I do know that many wubi users have fixed their wubi installs though using the below guide.

https://wiki.ubuntu.com/WubiGuide

Thank you for the tip
Well.. there's a part in this guide where it says:
"Ubuntu cannot be booted if Windows has not been shut down cleanly, you have to clear the Windows filesystem from Windows (there is no chkdsk equivalent for Linux yet). If Wubi fails to start, boot into Windows, run chkdsk /r from Windows on the same drive where you installed Ubuntu, shutdown cleanly and then try to boot into Ubuntu again. "

That's true. In fact, my Windows (7) doesn't "shutdown cleanly" because it has a problem that I never find a fix, where it doesn't shutdown. The system keeps in the "Logging off" screen, so I have to long press power.
But I ran the chkdsk and it still doesn't work!
I still can't boot ubuntu, can't even use that grub command line.
And the folders listed in the Guide (C:\ubuntu\disks\root.disk and C:\ubuntu\disks\boot) are there.

Any other suggestions?

You have to have a certain amount of space for Ubuntu installed through wubi.

Take your live CD and shrink your Win JerryLewis pro down that much.

Create an extended partition. Install Ubuntu on it as a dual boot instead of a file in MS.

Better yet just install Ubuntu using the "use entire disk" option. This will make sure you never have trouble with MS again. This will remove MS completely so you may be one of those folks that do not want to do this.