It appears that the system do not have a recovery partition, the only way to restore the system would be to use the recovery discs or operating system disc. Since you do not have any external USB CD/DVD drives, you can try the following options:
If you have a USB flash drive, then you can create a password XXXXX disc from your working computer, connect the flash drive to HP slate, try to boot from it and then reset your forgotten password. The steps to create a bootable flash drive is as follows
1 On your working computer, download ISO image file for UBCD from the following link
If you are unable to download the ISO image file from the above link, use the following link
2 Once the ISO file is downloaded, use the following guide to create a bootable flash drive
3 Now connect the flash drive to HP slate. Turn it on and press - (volume down) button to to enter bios setup. Press + button to select System Configuration, press Home.
Then goto Boot Options-> Boot Order. You will need to select your flash drive from the list of available devices using "+" and "-" buttons. To confirm your selection, press "home" button at the top of the device.
Once changes are made, press Ctrl+alt+del (security) button at the top of slate, select Exit saving changes, press Home
(volume up/down keys will act as arrow keys, home key will act as Enter)
Once it start from the USB flash drive you will see the following menu
4 Goto Parted Magic (using +, - button), press Home
5 Once parted magic starts, try to run PCLoginNow
The following link will provide a step by step guide on how to use PCLoginNow to reset password
Check the discs you received with your HP Slate. You probably have a Windows 7 installation disc. If so, you can create a USB flash drive from your working computer from which you can install a fresh copy of Windows on HP slate. The following guide will descrive how to setup the flash drive
Then connect the flash drive to HP slate, try to boot from it. Once it boots from flash drive, Windows setup will begin. And then re-install Windows
You can get a USB flash drive with automated system recovery from HP and use that flash drive to restore the system to factory defaults. For more information on this, please check the following link