Googled your system. HP reports it as being a pentium 4 at best and lists drivers for Windows 2000, XP and Vista. Windows 7/8 may work but I don't know. You can also try linux.
Anyway, get an operating system installation disc. You can either buy a Windows disc or download a ISO image and burn it to disc.
Change the boot order so the DVD-ROM boots first.
Insert the disc into the dvd drive and follow the installation instructions.