How do i get my Pentium D 925, 3.00 Ghz processor to clock at 3000 Mhz ?

After i reinstalled my Windows Vista Business OS, my Pentium D 925 3.00 Ghz processor will only clock at 2395.99 Mhz and not at the 3000 Mhz as it did before i reinstalled the OS. So how do i increase the clock speed on a HP Compaq Dx2300 Business computer back to 3000 Mhz ? You can't access the bus or clock speed in the Bios.