If under warranty have them replace the power adapter.

Also make sure you have the most current BIOS installed.

I already checked the BIOs... interesting that it came with A)5 and in the support site, latest version available is A04.

It is under warranty, so I'll call Dell.

I called Dell support. The tech guy came and replaced the motherboard and the power cable (inside the notebook, not from the power adaptor). Machine worked fine for 2 days, without displaying the message, but now it started to happen again.

I tried another power adaptor from Dell (of an older Vostro that I have), and same message appear :-(

Are you using a 90 watt OEM power adapter?

I really doubt they will give me a 90W adapter. For some reason, sometimes the notebook fails to recognize the adapter. My first guess would be to say the the adapter is bad, but I tested with another Dell 65W adapter, and same problem. To make things even weird, if I remove the adapter plug and insert it again, the error doesn't happen for some time.

Call dell and tell them you want the 90 watt power adapter that is compatible with your computer and see if that resolves your issue.

accessories.us.dell.com/.../category.aspx

No, I'm using the original 65W adaptador from Dell, that came with the notebook.