You need adb and fastboot on your comp (with the relevant dlls. Most probably you can find them on your phone's cd, or just google adb or 'android sdk').
Get your phone into fastboot, and connect it to your computer.
It is easier if you copy the img files to the same folder where fastboot.exe and adb.exe are. Open this folder in a command console, and type the following:
fastboot.exe flash recovery recovery.img
where 'fastboot-windows.exe' is the name of the fastboot.exe file,
'flash' is the parameter to flash the file to the phone,
'recovery' is the partition of the phone you would like to flash to,
and 'recovery.img' is the file you want to flash.
It is advised strongly to check the available parameters of fastboot (even on the internet) to make sure you will not do it on a wrong way, you should flash files to the correct partition (boot.img to boot partition, system.img to the system partition etc).