Hejsa
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Class\{4D36E96A-E325-11CE-BFC1-08002BE10318}\0000
Note that the "0000" key above might show as "0001", "0002" and/or "0003" on your machine, depending on your particular hardware settings. Select the key(s) appropriate to your case.
Right-click to create a new DWORD [REG_DWORD] Value, call it "EnableUDMA66" (no quotes), and type 1 in the Decimal box to enable ATA/66 (UDMA/66) support.
To disable it, change the Decimal value to 0, or delete "EnableUDMA66" altogether.
IMPORTANT: You also need to make sure the "EnableBigLba" DWORD [REG_DWORD] Value is turned ON under this Registry key:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Atapi\Parameters
If "EnableBigLba" is set to 0, run Regedit, right-click on it and modify the Decimal box to read 1.
This requires 48-bit LBA (Logical Block Addressing) motherboard BIOS support!
Reboot when done.
A MUST: To properly enable the UDMA/66 setting you MUST have your ATA/66/100/133 capable drive(s) hooked up to a SEPARATE IDE channel, OTHER than the one your older (E)IDE fixed/CD/DVD/removable drive(s) [even if ATA/33 (UDMA/33) capable] are connected to (if any)!