On switch on Tap F8
Choose repair my computer at the bottom.
Put in your username and admin password - cross fingers you have a re-install option at the bottom off HDBR. If not you will need media.
Ordering media from fujitsu can be ordered here: https://ts.fujitsu.com/recovery/
please use your serial number to check you are entiltled to order it.
No you cannot use his OEM 25 digit code key underneath on your own install cd.
it wont work because its OEM it will just tell you "invalid key"