Have you had to do a motherboard replacement or found your S/N details not showing up in the CMOS.
After entering the BIOS setup by hitting the F10 key whilst booting, press Ctrl+A to open additional fields under the Security tab, goto Security > System IDs menu.
- Enter Asset Tag ID: Enter your PC’s serial number
- Chassis Serial Number: same as asset tag
- Product Name: HP rp5800 Retail System
- Enter Feature Byte: ??
Save changes there and again when you exit the BIOS.