Will formatting HD and installing my own OS invalidate the warranty on laptops?

"no, installing a different operating system will not void the warranty. however, you may not be entitled to driver and software related technical support if you're using another OS.

and no, it would not be legal to use an OEM license with a virtual machine as it is technically speaking a different computer with different hardware (although virtualized)."

"While I (nor anyone else here unless they actually work for the companies in question) can gaurantee the accuracy of this answer. If you have a warranty question, it's best that you call the manufacturer and speak with a manager (the people answering the phones usually have a script and can't (or are afraid) to deviate from it).

That said, companies will LIKELY deny you certain support if you are not using the OS the computer shipped with. They COULD (and some will) tell you to reinstall the OS that came with the system and then they will help you. Certain things that are obviously OS independent and/or can be tested using manufacturer supported utilities will likely be fine (running a manufacturer supported boot CD to test the Hard Drive, for example. Other things, like why the screen won't run above 640x480, may require the original OS.

As for virtualizing the OEM license, it is prohibited. Microsoft doesn't permit OEM licenses to be run in virtual environments."